70 PALEONTOLOGY. 



ramus (fig. 18, 5), characteristic of the cretaceous strata and 

 oolites, differs from Perna chiefly in form, the larger valve 

 being sometimes completely involute, and resembling a 

 Nautilus. The genus Pinna, which appears to belong to this 

 family, although provided with two adductor muscles, occurs 

 fossil in the Devonian and all subsequent strata. Some of the 

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

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

 inch, resembling mineral masses of fibrous carbonate of lime. 



Amongst the mussel-tribe {Mytilidm) are many Silurian 

 species distinguished by their large, round, anterior muscular 

 scar (Modiolopsis, Hall), and others which have a straight 

 hinge-line and plaited valves (OrtJionotus, Conrad). Myalina 

 has the cartilage-groove repeated (fig. 17, 4), and is found in 

 the upper palaeozoic rocks. Sometimes the anterior adductor 

 is supported on a shelf, as in the recent Septifera and 

 Dreissena. True Mytili and Modiolw abound in the oolitic 

 strata. Dreissena, 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 Areadaz are far more numerous than the recent 

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

 a single species survives in the Coral Sea. The palaeozoic 

 Arks have anterior teeth like Area, and posterior teeth like 

 Cucidlwa, 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 farther distinguished by a deep furrow 

 behind the front muscular impression. These constitute the 

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

 and a prominent external ligament (fig. 17, 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. 1 8, 6). 



