226 



MOLLUSCA. 



More than one hundred fossil species have been described, 

 commencing in the Lias, and extending to the present day. 

 The Palaeozoic Modiola are probably referable to different gen- 

 era. The Date-shells (Lithodomus) form a sub-genus QiModiola, 

 and are distinguished by their habit of forming perforations 

 in rocks, in which they live. They appear to date from the 

 Lower Oolitic Rocks, and are known to palaeontologists by 

 both their shells and their burrows. 



The genus Modiolopsis (fig. 181), includes a number of 

 Silurian shells, the true place of which is somewhat uncertain. 



Fig. 181. Modiolopsis modiolaris. Lower Silurian. 



The shell is equivalve, very inequilateral, the beaks anterior, 

 and the surface smooth, or marked by fine concentric lines of 

 growth; The shell is thin, and its posterior end is consider- 

 ably broader than the anterior. 



The genus OrtJtonota likewise comprises a number of Silu- 

 rian Bivalves, and is also in a somewhat doubtful position. 



The shell (fig. 182) is elongated, 

 equivalve, very inequilateral, 

 having the beaks placed close 

 to its anterior end. The shell 

 is thin, and its margins are 

 parallel. 



FAM. 4. ARCADE. Shell 

 equivalve ; hinge long, with 

 many comb-like teeth ; muscu- 

 lar impressions nearly equal; 

 mantle - lobes separated ; foot 

 large, bent, and deeply grooved. The most important fossil 

 genera of this family are Area, Cucullcea, Pectimculus, Nucula, 

 Ctenodonta, Cyrtodonta, and Leda. 



The Arks (Area) have a straight hinge-line, with remote 

 beaks, separated from one another by an oval or lozenge- 



Fig. 182. Ortkoiiota paralleia. 

 Lower Silurian. 



