SILURIAN FAUNA. 145 



fauna, becoming practically extinct with the close of that period.* 

 It is a noteworthy circumstance in connection with the history of 

 this family that the more complicated or double-stemmed forms, such 

 as Diplograptus, Didymograptus, Phyllograptus, and Dichograptus, 

 preceded, in the order of appearance, the simple-stemmed forms, like 

 Monograptus and Rastrites, proving, contrary to what might have 

 been naturally supposed, that the latter were not the ancestral types 

 of the family. On any evolutionary hypothesis the simpler forms 

 appear to have been brought about as the result of degeneration. 

 In modern type hydrozoaus the Silurian, as all other Paleozoic, de- 

 posits are very deficient, a circumstance, doubtless, due in con- 

 siderable part to the perishable nature of the organisms belonging 

 to this class. The impressions of jelly-fishes have, however, been 

 indicated in both the Cambrian and Silurian rocks of Sweden. 

 Stromatopora, a very broadly distributed genus, whose affinities 

 are now generally conceded to be with the Milleporida, passes into 

 the Devonian formation. 



Of other Invertebrata, such as the echinoderms, mollusks, and 

 articulates, there is a vast profusion of forms, which, apart from 

 the mere matter of numbers, are in many respects sharply contrasted 

 with their predecessors of the Cambrian period. The brachiopod 

 mollusks, the predominant forms of which, as SiDirifer, Atrypa, 

 Athyris, Stropliomena, Rhynchonella, and Pentamerus, belong to 

 the group of the Bracliiopoda articulata, are structurally consider- 

 ably in advance of the inarticulate genera Lingula, Lingulella, 

 Discina, and Obolus, which make up almost the whole of the 

 corresponding Cambrian fauna; the latter, as far as is known, con- 

 tains but a single precursor of the articulate division, Orthis. The 

 prodigious development of the Silurian Cephalopoda would of itself 

 be sufficient to distinguish the period from the period iireceding. 

 While up to the present time only two species of this class, an 

 Orthoccras and a Cyrtoceras, are positively known from Cambrian 

 deposits, no less than eleven hundred species, referable to a very 

 considerable number of genera of Nautilidce — Orthoceras, Cyrto- 

 ceras, Gyroceras, Endoceras, Gomphoceras, Phragmoccras, Lituites, 

 Nautilus, &c. — have been described from the Silurian basin of Bohe- 

 mia alone. The total number of species of this jjeriod may be 



* The somewhat problematical Dictyouema passes into the Devonian; 

 Triplograptus, if a true graptolite, is Devonian. 



