LAMELLIBRANCHIATA. 



229 



In Trigonia (fig. 187) the shell is trigonal, with tubercles, 

 radiating ribs, or concentric ridges. The hinge-teeth are 

 two in one valve and three in the other. The Trigonics are 



Fig. 187. Trigonia scabra. Chalk. 



essentially Mesozoic, being only known, in the fossil con- 

 dition, as extending from the Trias to the Chalk. They are 

 not known with certainty to be represented in the Tertiary 

 Rocks at all; but the Australian seas have yielded some 

 living forms. 



The shells of the genus Myophoria have the umbones direct- 

 ed anteriorly, and are in most respects closely similar to Tri- 

 gonia. They belong exclusively to the Triassic period. 



The genus Axinus (Schizodus) is also nearly related to Tri- 

 gonia y but the shell is thinner, and is smooth, and the posterior 

 side is not so distinctly angular, but is marked by an oblique 

 ridge. The shells of this genus extend from the Upper 

 Silurian to the Trias, but are especially characteristic of the 

 Permian Rocks. 



FAM. 6. UNIONID/E. Shell usually equivalve, with a large 

 external ligament. Anterior hinge-teeth thick and striated ; 

 posterior laminar or want- 

 ing. Mantle-lobes united 

 between the siphonal ap- 

 ertures. Foot very large, 

 compressed, byssiferous in 

 the fry. All the members 

 of the UnionidcB are in- 

 habitants of fresh water, 

 and they are, therefore, not 

 known as fossils except in 

 fluviatile and lacustrine de- 

 posits. The only two fossil 

 genera of the family are 



TT M ' _. J A j Fig. 1 88. Unio Waldensis. Wealden 



Unto and Anodon. (Lower cretaceous). 



In the genus Unio (fig. 



1 88) the shell is oval or elongated, somewhat resembling that 

 of a mussel (hence the name of River-mussels commonly 



