724 



LAMELLIBRANCHIATA. 



and is bounded behind by an elevated ridge. The species of 

 Pleurophorus range from the Devonian to the Trias, but they are 

 most characteristic of the Permian formation. Possibly allied to 

 the above is the Matheria of the Ordovician rocks of Canada, in 



which the beaks are placed anteriorly 

 and the ligament is external; and the 

 Silurian genus Anodontopsis may, per- 

 haps, also find a place here. 



In this neighbourhood Dr Fischer 

 likewise places the extinct genera Myo- 

 concha and Hippopodium. In the 

 former of these the shell is thick and 

 mussel-shaped, with nearly terminal 

 beaks, and having the ligament external and contained in a 

 groove. The hinge is massive and curved, and in the right valve 

 carries a single oblique tooth. The anterior adductor impression 

 is deep, and the pallial line is entire. The species of Myoconcha 

 are essentially characteristic of the Secondary rocks, though the 

 genus has been stated to occur in the later Palaeozoic deposits. 

 In the genus Hippopodiu?n (fig. 600) the shell is oblong, massive, 

 and ventricose, with nearly terminal incurved beaks, an external 



Fig. 599. — Interior of the left 

 valve of Pleurophorus costatus, 

 from the Permian rocks. (After 

 King.) 



Fig. 600. — Hippopodium ponderosum. Lias. 



ligament, and an entire pallial line. The hinge carries in each 

 valve an oblique tooth, which becomes obsolete with age. The 

 genus is confined to the Jurassic rocks, a familiar species being 

 the Hippopodium ponderosum of the Lias. 



Family 6. Astartid^:. — In this family the mantle-lobes are free, 

 and the foot is long and pointed. The shell is equivalve, thick, 

 trigonal or oval, with a more or less distinct " lunule." The liga- 

 ment is external, and the hinge thick, with two or three cardinal 

 teeth in each valve, the lateral teeth being obsolete. There are 

 two adductor impressions, of which the anterior is the deepest, and 

 the interior of the shell is not pearly. The members of this family 

 are all marine, and the principal genera are Astarte and Opts. 



