192 GEOLOGICAL SUCCESSION OF ANIMALS. 



between the structure of an animal, and the epoch of its first 

 appearance on the earth's surface may be traced. 



472. As a general result of the inquiries hitherto made, it 

 may be stated that the paleozoic animals belong, for the 

 most part, to the lower divisions of the different classes. 

 Thus, of the class of Echinoderms, we find scarcely any 

 but Crinoids, which are the least perfect of the class. We 

 have represented, in the above sketches, several of the most 

 curious forms,* as well as of the Polyps, of which there are 

 some quite peculiar types from the Trenton limestone and 

 from the Black River limestone. 



473. Of the Mollusks, the bivalves or Acephala are nu- 

 merous, but for the most part belong to the Brachiopoda, 

 that is to say, to the lowest division of the class, including 

 mollusks with unequal valves having peculiar appendages 

 in the interior. The Leptcena alternata (b) which is found 

 very abundantly in the Trenton limestone is one of these 

 shells. The only fossils yet found in the Potsdam sandstone, 

 the oldest of all fossiliferous deposits, belong also to this 

 family (Lingula prima, a). Besides this, there are also 

 found some bivalves of a less uncommon shape (Avicula 

 decussata, e). 



474. The Gasteropods are less abundant ; some of them 

 are of a peculiar shape and structure (Bucania expansa,/; 

 Euomphalus hemisphcericus, c). Those more similar to 

 our common marine snails have all an entire aperture ; 

 those with a canal being of a more recent epoch. 



475. Of the Cephalopods we find some genera not less 

 curious, part of which disappear in the succeeding epochs ; 



* (i) Cyathocrinus ornatissimus, Hall ; (j) Melocrinus Amphora, Goldf. 

 (k) Cariocrinus ornatus, Say; (/) Columnaria alveolatv ; (m) Cyatho- 

 phyllum quadrigeminum, Goldf.; (n, o) Caninia Jlexuosa ; (p) Cheetetes 

 lycoperdon. 



