ASIPHONIDA. 479 



with prominent radiating ribs. The genus Cardiola (fig. 

 338, A and c) occupies an uncertain position, though the 

 type-species C. interrupta, of the Upper Silurian would 

 appear to be properly referable to the Aviculidce. The 

 genus, however, has been referred to the Arcadce ; and it is 

 questionable how many of the forms so-called truly belong 

 here. The shell in the type-form is oblique and equivalve, 

 with prominent beaks, and radiating surface - ribs, both 

 valves being convex, and the general appearance, as the 

 name implies, reminding one of the Cockles (Cardium). 



Closely related to the group we have been considering is 

 another section of the Aviculidce, of which Pterinea (figs. 336, 

 338, B) is the type. In this genus the shell resembles that of 

 Avicula in being eared and oblique, but it is nearly equivalve, 

 and the long straight hinge-line carries a few anterior radiat- 

 ing teeth along with elongated and oblique posterior teeth. 

 The genus ranges from the Silurian to the Carboniferous. 



Fig. 338. A, Cardiola interrupta ; B, Pterinea subfalcata ; c, Cardiola fibrosa. 

 (After M'Coy and Salter.) Silurian. 



Monopteria, separated from the preceding chiefly by its eden- 

 tulous hinge, is Carboniferous, as is the thin-shelled Pteronites 

 of the same formation ; while the Lower Silurian Megaptera 

 wants the posterior oblique teeth of Pterinea, and in other 

 respects approaches Ambonycliia. The Jurassic Pteroperna, 

 again, is a link between Pterinea and the group represented 

 by Perna. Lastly, the Carboniferous and Permian genus 

 Myalina may be regarded as allied to Pterinea by the 

 characters of its "hinge, while in its general form and nearly 

 terminal beaks it resembles the Mussels. 



A third group, hardly separable from the first, is the small 

 one represented by Vulsella and Malleus, together with some 



