Bkede. I Carboniferous Invertebrates. 51 



BRACHIOPODA. 



Uracil iopods are small animals with a two-valved shell, re- 

 sembling the clams somewhat in external appearance, though 

 they are very different in internal structure. They are in some 

 respects much more closely allied to the bryozoans, or sea-mats, 

 and worms, than to the clams. They are strictly marine 

 animals, living, as a rule, in rather deep water. Their distri- 

 bution, in this respect, has been divided into five zones, :i which 

 may be described as follows : Shore zone, or the beach between 

 high and low tide-marks ; the shallow-water zone, or water to 

 a depth of 90 feet ; the moderately deep zone, or water from 90 

 to 300 feet deep ; the deep zone, or water from 300 to 1668 feet 

 deep ; and the very deep zone, or water from 1668 to 17,670 feet, 

 or three and one-half miles deep. In each of these zones there 

 are species which do not occur in any of the others, while some 

 are common to two or more zones. There are, according to 

 Hall and Clarke, about 147 species of living brachiopods known, 

 which are distributed over the Atlantic, Pacific and Arctic 

 oceans, ranging from the island of Spitzbergen, north of Nor- 

 way, to Cape Horn, but being most abundant in tropical and 

 temperate waters. As a rule, they are distributed along coast- 

 lines and in the vicinity of islands. 



The living brachiopods are but a remnant of what was once 

 one of the most abundant and varied classes of animals of their 

 size that ever inhabited the earth. While the known living 

 forms are only 147 in number, the fossil species known at pres- 

 ent probably reach the enormous number of 6000, and of these, 

 upwards of 2000 are represented in the American rocks. They 

 are among the earliest fossils of which we have any record, and 

 culminated in the earlier part of geologic time. Tn the Cam- 

 brian, or earliest period of which we have any definite knowl- 

 edge of life, there are about 125 species known ; in the 



3. Thi< Introduction is Largely drawn from Eastman's Translation of Zittel's Handbook of 

 Paleontology, Schuchert's Synopsis of American PossU Brachiopoda (Bulletin of the U. S. 

 Geological Survey No. h;,, Paleontology of New York, vol. VIII. and Annual Report of the State 

 Geologist of New York for 1891, to which the reader i< referred for more detailed information. 



