ASIPHONIDA. 485 



ment, and having an oblique tooth in the right valve. The 

 Jurassic Hippopodium (fig. 346) typified by the well-known 



B 



Fig. 346. Hippopodium ponderosum. Lias. 



H. ponderosum of the Lias closely resembles the preceding, 

 but the shell is more massive and ventricose, and there is a 

 single thick oblique tooth in each valve. 



FAM. 4. ARCADE. Shell equivalve ; hinge long, with 

 many comb-like teeth ; muscular impressions nearly equal ; 

 mantle-lobes separated ; foot large, bent, and deeply grooved. 

 This large and important family has existed under many 

 and varied types from the Silurian upwards, and we may 

 briefly consider some of the leading genera under the three 

 groups of the Arcadce proper, the Nuculidce, and the Nucu- 

 lanidce, sometimes regarded as so many distinct families. 



The type of the first of these groups is constituted by the 

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

 beaks, separated from one another 

 by an oval or lozenge-shaped liga- 

 mental area (fig. 347). The teeth 

 are numerous and transverse, and 

 the surface is generally strongly 

 ribbed. Species of Area have been 

 described from the Lower Silurian 

 rocks upwards. It is probable, how- 

 ever, that the older Palaeozoic forms 

 referred here really belong to other 

 genera, especially to Tellinomya and 



Fig. 347. Arcaantiqua. Permian. 



Cyrtodonta. 



In Cucullcea the shell is trapezoidal and ventricose, and 

 the hinge-teeth are few and oblique, and at each end of the 

 hinge become parallel with the hinge-line. Species of this 



