(Fig. 114), the lobes being denticulated by secondary notches. In the Ammonites (Fig. 115) the saddes also, as well as the lobes, are denticulated in the most varied manner, notched, cut, or ramified, in form of branches, or foliated. The curvature of the suture line, as well as the formation of the saddles and lobes, takes place symmetrically in such a manner that a median line in the direction of the height divides the turns into two equal parts. The exterior lobe is called the external or siphonal lobe, when the siphon is on the exterior side. For Leop. von Buch it is the dorsal lobe, because he called this the back of the shell, but for recent authors, who consider the external side to be the ventral part, it is the ventral lobe. The opposite unpaired lobe is the internal lobe (or, according to opinions, antisiphonal lobe, or dorsal, formerly ventral lobe). Between the two are found the lateral lobes and the lateral saddles, situated on the body of the whorls, and the lobes and saddles concealed between the line of contact of the contiguous whorls and the internal lobe: among the former, the saddle which is found on the side of the external lobe is the external saddle, the two following are the first and second lateral saddles; all the others, up to the line of junction of the two whorls, are the auxiliary saddles; near the internal lobe is found, generally, an internal saddle, which is distinguished by its size from the other concealed internal auxiliary saddles. For the lobes, the first lateral lobe is that which is between the external saddle and the first lateral saddle; the following one is the second lateral lobe; all the others are called auxiliary lobes.

The beautiful researches of Hyatt and Branco have shown that the complicated lines of the suture of the Ammonites do not attain their normal form until the animal has developed a greater or less number of the chambers. The first sutures of all the Ammonites are always as simple as those of the Nautilidæ, Clymenias, or Goniatites (Figs. 112, 116); it is only little by little that the undulating lines become marked by secondary notches, and the complication of the line of the suture proceeds always from the exterior to the interior side. The complication of the suture line-which augments with age, so that the young sutures, more simple in Ammonites, resemble those of the geologically more ancient Goniatites and Nautilidæ, -shows, probably, that this differentiation indicates at the same time a perfection of the organism. It is truly difficult to discover wherein this consists. It is possible that the strongly ramified borders of the septa serve to increase the solidity (firmness) of the shells; for, in general, the shells of Nautilidæ, provided with simple suture lines, are considerably thicker than the shells of Ammonites-ordinarily as thin as paper. If one breaks cautiously, little by little, the enrolled shell of a Tetrabranchiate, there are distinguished the first whorls, and finally also the initial chamber of the whole coil. In the fossil evolute, or baculiform, shells this first chamber is, ordinarily, abbreviated or broken, and it is extremely rare that it is preserved.

According to Barrande, Hyatt, and Branco, there are two kinds of initial chambers in the Tetrabranchiates which can be distinguished by fundamental characters. In the Nautilus, and many of the paleozoic genera, the initial chamber is in the form of a truncated cone, slightly arched or straight, enlarged in front; upon the posterior convex wall, which terminates the truncated cone, is observed a depressed cicatrix, linear (Nau

tilus), circular (Cyrtoceras), elliptical (Trochoceras, Phragmocus), or some. times cruciform.

The initial chamber of Clymenia, the Goniatites, and the Ammonites is formed in an entirely different manner. In all these this spirally enrolled chamber has a vesiculous, spherical, or ovoid form, generally a little depressed and transversely striated; no scar or impression has been met with, and the siphon begins at the anterior wall. It is not probable that the initial chambers of the form of a truncated cone of the Nautilida are homologous with the spherical enrolled initial chambers of the Am、 monitida; on the contrary, the presence of a cicatrix makes it probable that this impression represents either the point of attachment, or the opening of communication, closed at a later stage, of a frail vesicle, perhaps membranous, which corresponds with the initial chamber of the Ammonites. According to this view, proposed by Hyatt, the initial chamber of the Nautilidæ should be equivalent to the second chamber of the Goniatites and the Ammonites.

The Siphon is a tubular prolongation of the skin of the posterior part of the body; it traverses all the chambers, and in Nautilus begins under the form of a closed tube covered with nacre, in the initial chamber, or truncated cone, where it touches the internal posterior wall at the same place, where exteriorly is seen the cicatrix. In the Ammonites and the Goniatites the siphon begins with a spherical swelling situated immediately behind the anterior wall of the initial vesicle (nucleus), consequently perforating only the first septum, without penetrating more deeply into the chamber. According to Hyatt, the part of the siphon penetrating into the embryonal chamber was, in general, only a depression of the first partition. Munier-Chalmas has observed in the Ammonites a particular prolongation of the siphon in the initial chamber which ought to have replaced the true siphon in the

embryonic stage; this prosiphon, as he calls it, is attached to the siphon, which begins in a reflected cul-de-sac, and is very variable in form. It forms sometimes an enlarged membrane, sometimes a cylindrical tube; the prosiphon does not communicate with the siphon, properly speaking.

In the recent Nautilus the siphon is a rather solid membranous tube covered exteriorly by a thin coating of brown color, earthy, formed of fine calcareous

grains. In the Ammonites (see Fig. 102) FIG. 109.-Median section of a young Am

monite (Trobites) showing the siphonal collar and its change of direction on passing from the initial chamber a, where it is retrosiphonate or turned backwards, to the third whorl, where it is prosiphonate or turned forwards. (After Zittel.)

this exterior calcareous envelope seems to take on a more substantial consistency, so that the siphon is enclosed in a delicate calcareous tube. It is necessary not to confuse this envelope of the siphon the siphonal collar, which is met with always where the siphon penetrates the septum.

itself with that which is called

The siphonal collar (Fig. 109) is a reflection or production of the septum of greater or less length, directed, generally, in Nautilus, backward,

and in Ammonites forward, and possesses the same structure with the septum. Ordinarily, the siphonal collar has only short length, and forms in front and behind the septum a sheath in the form of a band or collar about the siphon; but, sometimes, they pass from one septum to the other and form there a close continuous tube, or they have the form of an open funnel, slightly contracted behind, and prolonged to the next following septum, or even go beyond it, thus implanting themselves one within another (telescoping, Endoceras). The siphon is found in the median plane of the shell, and it is only exceptionally that it deviates a little from this plane. In this plane its position vacillates from the external side to the internal side in the different genera and the different species.

In the Ammonitidæ it is constantly on the external side of the shell. In the Nautilidæ its position does not remain constant in one and the same genus it may be external, internal, central, or intermediary.

Numerical Rate of Differentiation expressed in Terms of the Initiation of New Genera.-A study of the statistics of classification in relation to time will exhibit in this, as it has in previous cases, the grand features of the historical differentiation of the cephalopods.

First, we may consider what are the conclusions to be drawn from the succession of new genera as to the rate and order of the differentiations of the class Cephalopoda.

The classification itself is expressive of differentiation, as has been already observed. The division of the class into two orders is expressive of a very marked differentiation in structure. The genus is a group of organisms with the same ordinal and family structure, but exhibiting some particular characters, such as shape, relative size of parts, or special development of some part, which are the same for several different species; hence we recognize the number of genera tobe a numerical expression of the amount of differentiation attained in the family at any particular period of time, and the greater the number of genera in a particular family, at a. particular time, the greater is the amount of differentiation expressed in the family-history at that period, and the number of genera beginning or living in each period becomes a rough indication of the rate of expansion or evolution of the race under consideration. The total number of genera in the order Tetrabranchiata is 123 (Zittel). Two grand subdivisions of subordinal rank are made, including, respectively, Nautiloidea 29 genera, and Ammonoidea 94 genera. 28 genera of the 29 Nautiloidea had appeared in the Silurian.

One genus, Aturia, is considered to be a distinct new genus of the Tertiary; 16 genera were already well exhibited in the Lower Silurian, or Ordovician. Only 8 genera lived into the Devonian, only 5 to the Carboniferous, and but 2 (Orthoceras and Nautilus, the perfectly straight form and the tightly coiled form) survived from Paleozoic into Mesozoic time.

The other suborder, Ammonoidea, has 94 genera; of these, one genus is known as early as the Silurian (Goniatites), one new genus (Clymenia) was added in the Devonian, and in the latter part of the Carboniferous 5 more genera were initiated. Of the rest, all appeared in the Mesozoic, 41 genera beginning in the Triassic, 28 new genera starting in the Jurassic, and 18 new ones appearing, for the first time, in the Cretaceous. Not a single genus of the whole suborder survived the Cretaceous period. Thus the Nautiloidea are peculiarly Paleozoic in range, although there is still living the genus Nautilus, and the Ammonoidea are peculiarly Mesozoic, and every genus of this suborder is now extinct.

The other order, Dibranchiata, is less capable of showing its history: the hard parts were of inferior character and less in proportion to the fleshy parts, and upon the death of the animal were much more likely to be destroyed; 33 genera are known, and all are Mesozoic, or more recent. There were 3 genera in the Jurassic, 15, Triassic, 8, Cretaceous, 10, Tertiary and 3 now living.

Second. The lesson, regarding the evolution of the ordinal and subordinal characters and their generic expansion, which we derive from these statistics is as follows:

Rate of Differentiation of the Suborder Nautiloidea.—The Nautiloids (Orthoceras, Nautilus, and their kindred genera) first appeared in the Ordovician. Before the close of the Silurian this type had reached its fullest expansion, and began in a very marked manner to drop out of the race; 5 genera did not survive from Ordovician into Silurian, and of the 22 Silurian genera only 8 survived into the Devonian. Of this type the two genera to live the longest were Orthoceras, the simplest expression of the type, and Nautilus, probably the most differentiated; and the latter continued to live up to present

time. At least, of the structures preserved to tell us the story these two are the extremes-one, Orthoceras, a simple slender cone, straight, and with regular septa dividing it into chambers, and with a central siphuncle; the other, Nautilus, a closely coiled disciform shell, with siphuncle also central, M

FIG. 110.

.-Theoretic sections through the middle of the shells to show the variations in the curvature and coiling of Paleozoic Cephalopod shells. A, Clymenia; B, Nautilus; C, Nautiloceras: D, E, Aploceras; F, Orthoceras; G, Melia; H, I, Cyrtoceras; J, Gyroceras; K, Ophidioceras; L, Cryptoceras; M, Goniatites. (After Gaudry.)

outer chamber large, and whorls with ventral side out. The two features which best express in these shells the amount or degree of differentiation are, the amount and direction of the curvatures of the shells and the position of the siphuncles.