ASIPHONIDA. 



473 



ent family we have various forms, of which Anomia itself is 

 the type. This genus comprises Ostreiform Bivalves, with a 

 thin and translucent shell, which is fixed to some solid body 

 by a plug which passes through a hole or notch in the right 

 valve. The first appearance of Anomia seems to be in the 

 Cretaceous rocks ; but the Limanomia of the Devonian, the 

 Anomianella of the Carboniferous, and the Placunopsis of the 

 Jurassic, are earlier forms of the same type. The genus 

 Placuna, dating from the Tertiary (?), comprises flattened and 

 cake-like Oysters, allied in many respects to Anomia, but 

 having the shell unattached. 



A third group is that of the Pectinidce, often ranked as a 

 distinct family, and typified by the familiar genus Pecten, 

 comprising the Scallops. In this genus (fig. 329) the shell 



Fig. 329. Pecten Iskindicus, left valve. Post-Tertiary and Recent. 



rests upon the right valve, and the beaks are furnished with 

 ears. The anterior ears are usually the largest and most 

 prominent, and the shell is generally furnished with ribs 

 radiating from the umbos. There is a single, median, trian- 

 gular cartilage-pit. The right valve is the deepest, and is 

 notched below the anterior ear. Using the name Pecten in 

 its modern and restricted signification, it is probable that all 

 the Palaeozoic shells so named are really referable to allied 



