508 



LAMELLIBRANCHIATA. 



FAM. 19. ANATINID.E. Shell often inequivalve, with an 

 external ligament. Mantle-lobes more or less united. Siphons 



long, more or less united. Foot 

 small. The family of the Anatinidce 

 has considerable palseontological im- 

 portance, having a great development 

 in Secondary times, and appearing 

 to be also represented by numerous 

 types in the Palaeozoic formations, 

 though the true systematic position 

 of some of the old forms referred 

 here must be regarded as uncertain. 

 Of the more ancient types of the 

 family, the genus Myacites has a 

 gaping ventricose shell, with the 

 umbones directed anteriorly, and the 

 ligament external. The species are 



known in the Palaeozoic period, commencing in the Silurian ; 

 and they are represented in the earlier portion of the Secon- 

 dary period ; but they seem to have died out in the Chalk. 



Fig. 377. Anatina spatulata. 

 Kimmeridge Clay (Upper Oolites). 



Fig. 378. Palaeozoic Anatinidaj. A, Right side of Palanatina typa, showing the superior 

 size of the left beak Devonian (after Hall) ; B, Grammysia cingulata Upper Silurian ; c, 

 Allorisma (Edmondia) sulcata Carboniferous (after Phillips) ; D, Leptodomus truncatiis Upper 

 Silurian (after M'Coy). 



Palanatina (fig. 378, A) comprises certain Devonian types, 

 in which the left valve is somewhat larger than the right. 



