LAMELLIBRANCHIATA (il 



oolitic species, distinguished by the name Trichitcs, are inequi- 

 valve and irregular, and attain a thickness of more than an 

 inch, resembling mineral masses of fibrous carbonate of lime. 



Amongst the Mytilidm are many Silurian species distin- 

 guished by their large, round, anterior muscular scar (Modio- 

 lop&is, Hall), and others which have a straight hinge-line 

 and plaited valves (Orthonotus, Conrad). Myalina has the 

 cartilage-groove repeated (fig. 13, 4), and is found in the upper 

 palaeozoic rocks. Sometimes the anterior adductor is supported 

 on a shelf, as in the recent Septifera and Drcisscnia. True 

 Mytili and Modiolm abound in the oolitic strata. Drcisscnia, 

 now confined to the rivers of the Aralo-Caspian region, or only 

 naturalized in Western Europe, was represented by many 

 species, and some of large size, in the eocene of Hampshire and 

 miocene of Vienna, 



Fossil Arcades are far more numerous than the recent 

 shells, and mostly belong to the division Cucullcea, of which 

 a single species survives in the Coral Sea. The palaeozoic 

 Arks have anterior teeth like Area, and posterior teeth like 

 Ci'd'Hart, and differ from both in the reduction of the hinge- 

 area to a narrow tract corresponding with the posterior half 

 only in the recent shells. The casts of Ark-like shells in the 

 Silurian rocks are further distinguished by a deep furrow 

 behind the front muscular impression. These constitute the 

 genus Ctenodonta (Salter), which has hinge-teeth like Kucula, 

 and a prominent external ligament (fig. 13, 5). Some of the 

 oolitic Arks, with a byssal sinus, and the posterior teeth very 

 long and parallel, form a sub-genus called Macrodon (fig. 14, 6). 

 Others, with prominent umbones, teeth like Kucula, and a 

 striated ligamental area, form the genus Isoarca of Minister 

 (fig. 14, 7). Above 200 species of Kucula and Lcda are 

 known only as fossils, and range through all the rock systems. 

 The palaeozoic species are anomalous in form, and when better 

 understood, will certainly be considered distinct as genera. 



