UPPER SILURIAN. 259 



and the Conehifers, with the exception of the Cardium family, have 

 thepallial impression entire (fig. 163). The species of Conehifers are 

 mostly of the Mytilus, Avicula, Area, and Cardium families ; those of 

 the Gasteropods, mainly of the Bellerophon and Trochus families. 

 The Tentaculites have their climax in the Upper Silurian, occurring 

 in great numbers in some of the rocks ; after this they are compa- 

 ratively rare. Among Cephalopods, the Orthocerata, while common, 

 are not so large nor so numerous as in the Lower Silurian. 



The genus Ormoceras — with large beaded siphuncle — ceases with 

 the Niagara period. Both the straight and the curved or coiled 

 shells have the partitions simply arched, and not plicate as in 

 after-time. The Conularice are more numerous and larger than 

 before. 



(d.) Articulates. — The sub-kingdom of Articulates still embraces 

 only the water-types of Worms and Crustaceans. Trilobites are 

 multiplied in genera, — Homalonotus and Phacops being added to 

 Calymene, Agnostus, Asaphus, Illcenus, Lichas, Acidaspis, and Dalmania 

 of the Lower Silurian. The bivalve Crustaceans, or Ostracoids, are 

 very common. In the Eurypterus (fig. 433) there is a new step in 

 the development of the Crustacean type, yet one still within the 

 lowest order, that of Entomostracans, It was observed, p. 203, 

 that Trilobites and Phyllopods were comprehensive types, — that is, 

 types embracing features of other unexpressed groups of Crusta- 

 ceans, and not typical Entomostracans. The Ostracoids, also, are a 

 peculiar group, and have a close resemblance in general structure 

 to the young of a tribe that appears long afterwards, — the one 

 including the Cirripeds (Barnacles and Anatifas, p. 154). With the 

 Eurypterus commences the typical Entomostracan. It belongs to 

 the same general group with the Cyclops and Sapphirina, — the 

 Cyclopoids, — the largest tribe in the order of Entomostracans. 



(e.) Extinction of species. — The number of Upper .Silurian species 

 thus far described from the American rocks is about 680, which is 

 at least 200 short of the number existing in collections. Not a 

 species existed in the later half of the Upper Silurian that was 

 alive in the later half of the Lower Silurian. Less than a dozen 

 species are continued into the Devonian, and 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 

 the few before enumerated, Lingula, Discina, Nautilus, Rhynchonella, 

 Pleurotomaria, and Crania, become extinct. 



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



