ANATINACEA. 



749 



or less united ; there are long siphons, more or less extensively con- 

 joined, and the foot is of small size. The shell is thin, usually 

 nacreous internally, and commonly granulated externally, and as a 

 rule somewhat inequivalve. The ligament is wholly or partially in- 

 ternal, usually in a spoon-shaped cartilage-pit, flanked by one or two 

 cardinal teeth, and often containing a detached ossicle. The pallial 

 line is usually sinuated. All the members of this family are inhabi- 

 tants of the sea. 



In Anatina itself (fig. 635) the shell is oblong, very thin, gaping 

 behind, and having the beaks turned towards the posterior side of 

 the shell, which is more or less attenuated. 

 The hinge of each valve carries a spoon- 

 shaped cartilage-process, and the beak is 

 usually supported by an oblique, back- 

 wardly directed internal ridge. The pal- 

 lial line is deeply sinuated. The earliest 

 undoubted forms of Anati?ia are found 

 in the Lower Cretaceous rocks, and the 

 genus survives at the present day. The 

 genus Thracia, ranging from the Trias to 

 the present day, is in most respects very 

 similar to A?iati?ia, but the shell is inequi- 

 valve (the right valve being larger than 

 the left), and there is a short external 

 ligament, in addition to the internal car- 

 tilage. In Pandora the shell is also thin 



and inequivalve, but it does not gape behind. The species of this 

 genus range from the Eocene to the present day ; and the species 

 of the allied genus Lyojtsia have a similar range in time. Lastly, 

 the Cretaceous genus Liopistha appears to belong here, though the 

 pallial line is apparently not sinuated. Numerous Palaeozoic Bivalves 

 have been referred to the Anatinidce, but most of these are character- 

 ised by an entire pallial line and an external ligament, and may be pro- 

 visionally placed in the family of the Grammysiidce. It is possible, 

 however, that the genus Allorisma (fig. 636, c) should find a place 

 in the Anati?iid<z, with which it agrees in the possession of a sinu- 

 ated pallial line, and in having the external surface of the valves 

 granulated. The shell in this genus is transversely elongated, equi- 

 valve, with anteriorly-placed, almost terminal beaks, and a concen- 

 trically-striated surface. The hinge is edentulous, and an external 

 ligament is present. The species of Allorisma range from the 

 Devonian to the Permian rocks. 



Family 4. Grammysiidce. — This family comprises a number of 

 Palaeozoic Bivalves which differ from the typical Anatinidce in hav- 

 ing an entire pallial line, and also in the fact that the ligament is 



Fig. 635. — Anatina spatulata. 

 Kimeridge Clay (Upper Oolites.) 



