§ 20. SILURIAN GASTEROPODA AND CEPHALOPODA. 787 



in the oldest deposits ; but a bracliiopodous shell with free 

 valves, and without an articulated hinge, is the lowermost 

 term of animal life, according to our present paheontological 

 knowledge. A species of Ungidite or Obolus (a small 

 orbicular horny shell) occurs in the inferior limits of the 

 fossiliferous deposits in Russia and Europe ;* occupying the 

 same geological position as the Lingulae, in the American 

 and Silurian beds above cited. 



Of the pteropodous mollusca,f there are several species 

 belonging to two genera, named Comdaria, and Creseis.\ 



20. Silurian Gastekopoda and Cephalopoda. — Of 

 the Gasteropoda or simple univalve mollusca, the Euom- 

 phalus (ante, p. 736) appears to be the most ancient type, 

 and several species are common both in the Silurian and 

 Devonian formations ; but these are associated with shells 

 of the existing genera of Turbo, TurriteUa, Trochus, Pleu- 

 rotomaria, and Natica. 



Cephalopoda. The cephalopodous mollusca are repre- 

 sented in the Silurian deposits by several genera, of which 

 the Orthoceras, and allied forms, are the most nume- 

 rous and characteristic. Goniatites and JBellerophon, so 

 frequent in the Devonian and Carboniferous formations, 

 though present, are comparatively rare. Of the Ortlw- 

 cerata upwards of thirty species of the genus Orthoceras 

 have been obtained from the British strata. The Actino- 

 ceras, distinguished from the former by the remarkable 

 structure of the siphunculus, the tube of which is sur- 

 rounded by verticillate processes, has hitherto been observed 

 in the Silurian deposits only, occurring in the limestone of 

 Lake Huron, and in that of Ireland. § 



* Geology of Russia, vol. ii. p. 292. 

 + Medals of Creation, p. 367. 



J Quarterly Geological Journal, vol. i. p. 146, contains a notice on 

 some fossil species of Creseis. by Professor Forbes. 

 § See Mr. Lyell's Elements of Geology, p. 174. 

 VOL. II. 3 F 



