It is beyond my purpose to attempt to follow the more recent history of these plants. It has been my object to assemble data concerning an important part of the botanical results of the Lewis and Clark Expedition and to add another chapter to the gradually accumulating record of that great undertaking.

BOTANICAL LABORATORY,

UNIVERSITY OF PENNSYLVANIA.

FOSSIL AND RECENT CORALS AND CORAL REEFS OF WESTERN MEXICO. THREE NEW SPECIES

By R. H. PALMER

IT is a matter of more or less common knowledge that along the western coast of the Americas there is an entire absence of extensive coral reefs and a paucity of coral fauna, both of which are so characteristic of the tropical and subtropical waters of the West Indies and adjacent islands.

This has been explained by the relatively warmer water of the east coast as compared with that of the west coast, the difference of temperature being attributed to the long exposure of the equatorial current to the tropical sun as it passes westward across the Atlantic. Although the assumed relative temperatures of the waters of the two coasts may be true as a generalization it is at variance with observations made along the Oaxaca coast of southern Mexico, where the water has a winter temperature of 28° C. and a summer temperature of 29° C. In Tortugas Bay on the Florida coast, where corals. abound, Vaughan reports 1 24.4° C. as the average January temperature over a period of eleven years and a July temperature of 28.7° C. during the same length of time. The same author states that vigorous reefs will endure a temperature of 18.15° C. or nearly 10° lower than the waters along the Oaxaca coast. The other factors involved in coral control are left for later discussion.

While reef-forming genera are present on the west coast they never assume the importance of their West Indian relatives. That this is a constant feature is indicated by a few glimpses into the geologic past that are afforded by the rock record.

The first known occurrence of corals in any abundance is in the Cenomanian rocks that are exposed in the States of

1 1 Vaughan, T. W., Carnegie Inst. Wash. Pub. 213, pp. 319–329, 1918.

Puebla, Jalisco and Colima. Though the latter two localities are on the present Pacific slope, the fauna is entirely Atlantic in its relationship. In Puebla, these Cretaceous corals belong for the most part to reef-forming genera. So abundant are the remains that during life they must have built reefs of large

extent.

Throughout the remaining Cretaceous no coral reefs and but few corals are known. The total absence of Pacific Cretaceous fauna in southern Mexico precludes any statement as to the Cretaceous corals of the Pacific Ocean.

Atlantic Eocene is known in southern Mexico but no corals have been reported from it. It is probable, however, that they occur, as Eocene coral reefs of some importance are known in the West Indies. On the Pacific side, the Eocene is not known.

During the Oligocene reef-building corals reached a degree of abundance along the shores of eastern Mexico never before nor since equalled. In the Tampico Embayment there are, in the San Rafael formation, extensive beds composed almost exclusively of corals. These beds are so characteristic and of such wide extent that they are valuable for correlation purposes. On the Pacific shore the Oligocene is also absent.

On the Isthmus of Tehuantepec there are rather extensive exposures of Upper Miocene and Pliocene with Caribbean fauna. In these a few corals of the solitary type occur but no reef-builders are known. No Miocene or Pliocene is known on the west coast of southern Mexico.

Curiously enough, near the head of the Gulf of Lower California there is a Miocene or Pliocene reef. The corals of this reef all belong to living genera of the Caribbean Sea.

Pleistocene corals occur in lately uplifted beds in the vicinity of Tampico. No reefs, however, have been reported.

Along the Oaxaca coast the Pleistocene occurs as a low cliff of loosely consolidated beach material. It extends along the coast in patches for some twenty-eight miles. In some places it is on the beach and in others it is inland about a mile and a half. Its elevation varies from low tide level to fifty

feet or more. No reefs have been found in this exposure although there are abundant coral remains which include such reef-forming genera as Porites and Pocillopora.

Recent corals occur both along the Atlantic and Pacific coasts of Mexico. With the exception of a few genera that are nearly world wide in distribution, the coral faunas of the two coasts are quite distinct, indicating, I might add in passing, a somewhat lengthy separation. Curiously enough, corals are much more common along the western coast than along the shores of the Gulf of Mexico. It is possible that the explanation of this phenomenon is the same as that of the general paucity of corals along the west coast as compared with the islands of the Gulf of Mexico and Florida waters in general. It appears that this explanation is based on ecological factors that are under coast control. Corals are sessile animals and hence require stability to the base to which they are attached. This condition is best found along rocky shores where the beach sand is limited in amount or does not exist.

Sandy beaches and sandy bottoms are particularly unfavorable to either crawling or burrowing or sessile forms. Long stretches of beach sand with only an occasional beach worn shell bear ample witness of this fact. Here the soft shifting sands afford not only no foothold but subject any organism that does not swim to periodic burial which if sufficiently prolonged, is fatal and, what is equally destructive to a coral that cannot close itself up, to actual choking by the sand grains entering into the vital parts.

Practically the entire eastern coast of Mexico is a low lying plain that is but a few feet above sea-level. Long sandy beaches and bars extend from Vera Cruz almost continuously to the Rio Grande. Rocky coasts are absent. It is true that there are several areas of elevation but only the soft Tertiary rocks are exposed and they are nearly as hostile to shore life as the sandy beaches.

On the west coast, however, a different geological history has been followed by different shore conditions, hence different ecological conditions obtain.