ARCACEA. 



715 



present, while the foot is discoid, and there are two nearly equal 

 adductors. The shell is equivalve, with an internal or external liga- 

 ment, but without a ligamental area ; and the hinge is furnished 

 with numerous narrow teeth. The pallial line may be entire or 

 sinuated. This family includes two related groups, typified respec- 

 tively by Nucula and Nuculana (Zeda), the former group being char- 

 acterised by the fact that the pallial line is entire and the posterior 



Fig- 585. — Cyrtodonta Hindi. (Billings.) Ordovician. a, Side view ; b, Dorsal view, 



side of the shell is short, while in the latter the pallial line is more or 

 less sinuated and the posterior side of the shell is long. 



In the genus Nucula itself (fig. 587) the shell is trigonal or oval, 

 and the beaks are reversed and turned towards the posterior side of 

 the shell, which is also the shortest side. The interior of the shell 

 is nacreous and the ventral margin may be smooth or finely dentic- 

 ulated. The hinge is angulated, and shows a central internal carti- 

 lage-pit, flanked on each side by numerous teeth. The Secondary 

 and Tertiary rocks have yielded a considerable number of species of 

 Nucula, and there are many living forms. The Palaeozoic shells 

 which have been referred to Nucula probably belong really to other 

 types (Tellinoviya, <xx.) 



In the neighbourhood of the preceding may be placed the Silurian and 

 Devonian genus Cucullella (fig. 589, b), in which the hinge-line is straight 



