MYTILACEA. 



705 



notch. The genus begins in the Trias, is well represented in 

 the later Secondary and Tertiary deposits, and still survives under 

 a few forms. A large and well-known species is the Perna Mulleti 

 of the Neocomian rocks. Allied to the preceding is the genus 

 Gervillia (fig. 570, a), in which the shell is elongated and very 

 oblique, with a broad and wing-like posterior ear. The hinge-line 

 is furnished with numerous cartilage-pits (fig. 570, b), and the 

 anterior ear is comparatively small, while the beaks are nearly or 

 quite terminal in position. The species of Gervillia abound in the 

 Triassic, Jurassic, and Cretaceous rocks, and a single species of the 



Fig. 570. — Types of Avicididce. a, GcTi'illia Hartmanni — Lias; B, Part of hinge of the same, 

 enlarged; c, Hoernesia J oarmis- Austria, slightly enlarged — Trias; d and e, Cassianella gry- 

 plieata, of different ages — Trias. (After Miinster and Laube.) 



genus is found in the Eocene rocks. In the Trias are found various 

 forms of the closely allied Hoernesia (fig. 570, c), in which the left 

 valve is greatly inflated, with a strongly incurved beak, while the 

 hinge-line is crenulated, and a single strong tooth exists in both 

 valves. Bakewellia, of the Permian, resembles Gervillia in general 

 form, but the hinge is provided with anterior and posterior teeth, 

 and the cartilage-pits are few in number. The Devonian genus 

 Actinodesma is probably a still older relative of Gervillia. 



Nearly related to both Gervillia and Perna is the important and 

 widely-distributed genus Inoceramus (figs. 571, 572), which is entirely 

 confined to the Secondary period, and is mainly characteristic of the 

 Cretaceous series. The shells of this genus are inequivalve, with 



vol. 1. 2 Y 



