ably represent in only the most meagre way the forms of these classes which lived in past ages. Figures 21, 22.

FIG. 22.-Arthropod. A, Pterygotus anglicus Ag. Devonian. Dorsal view. B, Pt. osiliensis Schm. Silurian. Under side of head. = cephalic shield; thorax ; s= abdomen; a= eyes; eyelets; - cephalic appendages; 1-6 thoracic segments; 7-13 = abdominal segments; t = terminal segment or telson; ep epistoma; kl masticating plates of the sixth pair of appendages; m = metastoma; z = median plates; = median suture.

Molluscoidea. The Molluscoidea, including the Polyzoa and the Brachiopoda, is a group of much interest to the Paleontologist. The Brachiopods are well preserved, and are, perhaps, from the point of view of the scientific paleontologist, the most important group of animals he is able to study. Of their history, the record is more complete, the condition, as a whole, more perfectly preserved, the missing links fewer, than for any other group. They have been studied more thoroughly, are of greater value as marking geological horizons, probably, than any other. They develop a chitonous or calcareous bivalved shell, the external and internal form of

which, and the intimate structure of the shell substance, are generally well preserved. Figures 23, 24.

FIG. 23.-Molluscoida, Bryozoa. A, B, Fenestella retiformis Schl. Permian. A, a funnel-shaped stock from the outside. B, enlarged view showing the cell mouths (o) and the perforations (2) between the cell-rows of the conccium. C, Archimedes wortheni Hali. Carboniferous. The stock consists of a broad conccium (b), wound spirally about a central axis (a). Fragments of the conccium separate from the axis present a structure similar to that of Fenestella.

FIG. 24-Molluscoida, Brachiopoda A, B, C, Inarticulata. Lingula. A, L, anatina Brug., living, pedicle valve from within. st = pedicle; s, d, a, s', muscular impressions. B, L. tenuissima Br. Triassic. C, L. Beani Phill. Jurassic. D, E, Brachiopoda articulata. Atrypa reticularis L. sp. Devonian. D, surface view of brachial valve. E, view of interior, the brachial valve being in great part removed; foramen for passage of the pedicle; cr = crura; b = jugal processes or jugum ; sp = spires or spiral coils of the brachidium.

Mollusca. Of the true Mollusks, all the four classes, Lamellibranchiata, Gastropoda, Pteropoda, Cephalopoda, construct, in most of their genera, calcareous or horny shells, external or internal, which are preserved, more or less perfectly, in a fossil state. Gastropods and Lamellibranchiates in the older rocks are very apt to be in the condition of impressions and moulds, the substance of the shell being dissolved and carried away; this is also the case with many

families of the other two classes, so that very much is wanting to a complete record of these classes. Figures 25, 26, 27.

FIG. 25.-Mollusca, Lamellibranch. Venus multilamella Lmk. Tertiary. A, a right valve, outer surface; lu lunula. B, the same interior. C. hinge of the left valve. ' anterior, n' = posterior, muscular impressions; mb = pallial sinus;= ligamental pit; 2 = cardinal teeth. FIG. 26.-Gastropod. A, Paludina pachystoma Sdb. Tertiary Miocene. B, P. avellana Neum. Pliocene.

FIG. 27.-Cephalopod. Ceratites nodosus d. Haan. Triassic. A, complete shell from the side; B, front view of the same; mr rim of the outer chamber; ss, ss1, ss, hs = saddies of the sutures; el, sl, sl, hl= serrate lobes of the suture lines.

Vertebrata. Of this branch there is scarcely an order that does not develop hard parts of some kind, which might be preserved in fossil condition under favorable circumstances. Among the lowest orders (Lancelot, Hag-fish, Lampreys) there is nothing likely to be preserved, except small teeth. In the cartilaginous fishes teeth are the main parts of sufficient hardness to resist decay and disintegration, while the

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bones and scales of other fishes are hard and enduring if well
buried under water, but are easily destroyed if left exposed in
contact with the atmosphere for a long
time. So again, while many fish and
reptiles and a few mammals are in-
habitants of the ocean, birds and most
mammals and many reptiles are in-
habitants of land, and many fish and
reptiles are fresh-water species only.
Again, the remains of Vertebrates are
subject to the destructive agency of
lower animals and of themselves, so
that it is not to be supposed that under
the most favorable natural conditions

1709

FIG. 30.-Vertebrate, Reptile. Ichthyosaurus quadriscissus Quenst. Jurassic. Skeleton of a
young individual
A coprolite. (After Steinmann and Doederlein.)

anything more than the most meagre representation of the vertebrate life of the world would be preserved in fossil condition, and of those preserved, the more abundant would be reptiles, fishes, and larger mammals, with a few birds. (Figures 28-32.)

Looking over the Animal Kingdom, in this general way,

FIG. 31.-Vertebrate. Saurura. Archeopteryx macrura Owen. Jurassic. Restored in the position of the Berlin specimen. c = carpus; cl clavicula?; co coracoid; = humerus; = radius; s= scapula; = ulna; I-IV = 1-4 fingers. (After Steinmann and Doederlein.)

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