UPPER SILURIAN. 253 



first species of that division of the genus Spirifer in which the ribs of the shell are 

 bifurcated; a kind that afterward became common. 



The Lamellibranchs and Gasteropods were few, compared with Brachiopods; and, in 

 both groups, the species were mostly siphonless; that is, the Gasteropods had the aper- 

 ture without a beak, and the Lamellibranchs, with the exception of the Cardiuin 

 family, had the pallial impression entire (Fig. 153). The species of Lamellibranchs 

 were mostly of the Mytilus, Avicula, Area, and Cardiuin families; those of the Heter- 

 opods and Gasteropods, mainly of the Bellerophon and Trochus families. 



The Tentaculites had their climax in the Upper Silurian, occurring in great numbers 

 in some of the rocks; after this, they were comparatively rare. 



Among Cephalopods, the Orthocerata, while common, were neither so large nor so nu- 

 merous as in the Lower Silurian. The genus Ormoceras — with large beaded siphuncle 

 — ceased with the Niagara period. Both the straight and the curved or coiled shells 

 had the partitions simply arched, and not plicate as in after-time. 



The Conularim were more numerous and larger than before. 



The subkingdom of Articulates, as far as knowledge from fossils goes, still em- 

 braced only the water-types of Worms and Crustaceans. Trilobites were multiplied in 

 genera, — Homahnotus, Phacops, and others being added to Calymene, Aynostus, Asa- 

 phus, Illaniis, Llchas, Acldaspls, and Dalmanites, etc., of the Lower Silurian. The bi- 

 valve Crustaceans, or Ostracoids, are very common. In the Eurypterus and Pterygotus 

 (Fig. 461), there is a new step in the development of the Crustacean type, and, in the 

 Ceratiocaris, an advance in still another direction. Yet the class of Crustaceans had 

 not made progress beyond its lowest order, that of Entomostracans, — except it be 

 that the earliest group, that of Trilobites, overstepped these bounds at the very begin- 

 ning (p. 123). The Ostracoids were precursors of the modern Ostracoids; the Cera- 

 tiocarids, of the modern typical Cyclopoids; and the Eurypterids, of other Cyclopoids 

 of flattened forms. But while precursors in time, many of the species of these groups 

 were gigantic compared with the largest of those of the present day, exceeding the 

 latter ten to fifty times in lineal dimensions; and it is not easy to prove that the 

 smaller moderns are in any way their superiors in grade. While the middle of the 

 Silurian age was the time of greatest expansion for the group of Trilobites, the closing 

 period of it, with the early Devonian, appears to have been the time of culmination of 

 the Entomostracan order. 



Extinction of species. — The number of Upper Silurian species thus far described 

 from the American rocks is about 3,500, which is at least 500 short of the number 

 existing in collections. There is no evidence that a species existed in the later half 

 of the Upper Silurian that was alive in the later half of the Lower Silurian. A num- 

 ber of species are continued into the Devonian ; but these disappear long before the 

 close of that age. 



Genera of existing seas. — To the list of existing genera, no additions are made in the 

 course of the Upper Silurian. All but Linyula (?), Discina, Nautilus, Rhynchonella, 

 Pleurotomaria, and Crania, become extinct. 



Climate. — There is no evidence that the climate of America in- 

 cluded frigid winds or seas. The living species in the waters between 

 the parallels of 30° and 45° were in part the same with, or closely- 

 related to, those that flourished between the parallels of 65° and 80°. 

 (See pages 221 and 230.) From this life-thermometer we learn only 

 of warm or temperate seas. 



