SIPHONIDA. 509 



Grammy sia (fig. 378, B), of the Upper Silurian, has an elon- 

 gated shell, resembling the preceding in general form, but 

 having a single or double fold extending from the beaks to 

 the middle of the ventral margin. Leptodomus (fig. 378, D) 

 and Sanguinolites, both characteristically Upper Silurian, but 

 the latter extending to the Carboniferous, comprise thin, 

 oblong or elongated, concentrically-striated shells, apparently 

 referable to this family, but for the most part very imper- 

 fectly understood. The former is stated to possess a well- 

 defined " lunule" beneath the beaks. Allorisma (fig. 37$, c) 

 and JEdmondia, characteristic of the Carboniferous and Per- 

 mian rocks more particularly, though not unknown in the 

 Upper Silurian and Devonian, likewise comprise thin, equi- 

 valve, concentrically - striated shells, with anterior beaks. 

 Oardiomorpha, ranging from the Silurian to the Carbonif- 

 erous, as its name implies, possesses a shell with the general 

 form of the Cockles, but smooth or concentrically-striated, 

 the beaks being prominent, and the hinge edentulous. Lastly, 

 we may place here the Carboniferous genus Anthracomya 

 ( = Naiadites), in which there is a thin concentrically-striated 

 oblong shell, with anterior beaks, and an external ligament. 



In the Lantern-shells (Anatina) the shell is oblong, gaping 

 posteriorly, and having the beaks directed towards the pos- 

 terior side (fig. 377). The hinge of each valve carries a 

 spoon-shaped cartilage-process. The Anatince are doubtfully 

 represented in the Devonian, and still more dubiously in the 

 Silurian rocks. They occur, 

 however, abundantly in the 

 Secondary rocks, and are 

 present in smaller numbers 

 in the Tertiaries. Nearly 



related tO the preceding is Fig. 379. Pholadomya cequivalvis. Chalk. 



the genus Thracia, which 



is first certainly known to make its appearance in the 



Jurassic rocks. 



The genus Plwladomya includes a large number of shells, 

 which are equivalve, oblong, and gaping posteriorly (fig. 379). 

 The shell is thin, ventricose, and adorned with radiating ribs 

 on the sides. The ligament is external, and there is a large 



