Handbook of Paleontology 311 



The smooth-shelled, biconvex Whit field ellas are present. 

 The Spirifers appear almost abruptly in the Clinton 

 (Middle Silurian). They are still for the most part small 

 forms though there are a few large ones {S. niagaraensis, 

 S. radiatns). Rhynchonelloid (Camarothoecia, Rhyncho- 

 treta) and pentameroid (Pentamerus) shells are abun- 

 dant and there are some specialized forms (Trimerella) . 

 Pelecypods are in general less distinctive. There are 

 many types of gastropods, both low and high-spired and, 

 especially in the Upper Silurian, they are often strongly 

 ornamented. Among the cephalopods the most charac- 

 teristic forms are the ringed orthoceran types (Daw- 

 sonoceras) and curved forms with contracted aperture 

 (Phragmoceras, Gomphoceras). There are also many 

 forms with spiral coil (Trochoceras, etc.) The develop- 

 ment of cephalopods (nautiloids) in European seas is 

 much greater. The conularids are represented by the 

 genus Conularia and the pteropods by Tentaculites so 

 characteristic of our Manlius. Among echinoderms, 

 crinoids and cystoids are abundant. Crinoids are better 

 developed than in the Ordovician and are so numerous 

 that their stems and "joints" constitute an important part 

 of certain limestone beds. About 400 forms have been 

 described from the American Silurian. Characteristic 

 forms among the crinoids are Ichthyocrinus and Enca- 

 lyptocrinus; among the cystoids, Caryocrinus and Callo- 

 cystis. Blastoids (Troostocrinus) are present, but they 

 are rare in both the Silurian and the Devonian. Other 

 echinoderms are rare. Trilobites are still numerous, but 

 much less so than in the Ordovician. They are, for the 

 most part, species of genera which have survived from 

 the preceding period. Over 100 species have been de- 



