490 LAMELLIBRANCHIATA. 



is more familiarly called, Leda (fig. 353, A), the shell re- 

 sembles that of Nucula, but is rounded in front and pro- 

 duced behind, while the pallial line is slightly indented. 

 The hinge has numerous small teeth on either side of a 

 small central cartilage -pit. Various species of the genus 

 have been described from the Palaeozoic rocks, and it is 

 abundantly represented in the Secondary, Tertiary, and 

 Post-Tertiary deposits. Yoldia (fig. 353, c and D) resembles 

 the preceding in most respects, but the teeth are comb-like, 

 and there is a large pallial sinus. The genus seems to be 

 represented as early as the Devonian or Carboniferous, but 

 can hardly be distinguished from Nuculana in the fossil con- 

 dition. Lastly, the Silurian and Devonian genus Cimdlella 

 (fig. 353, B) differs from the preceding chiefly in the fact 

 that there is a long internal septum extending from below 

 the beak towards the anterior muscular impression, while the 

 pallial line is said to be entire. 



FAM. 5. TEIGONIAD^E. Shell equivalve, trigonal; hinge- 

 teeth few, diverging ; umbones directed posteriorly. Mantle 

 open ; foot long and bent. The most important genera of 

 this family are Trigonia, Myophoria, and Schizodus. 



In Trigonia (fig. 354) 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 Tri- 

 gonice are principally 

 Mesozoic, being abun- 

 dant and characteristic 

 fossils of the Secon- 

 Hg.s54.-zv*ffonia6m. chalk. dary deposits from the 



Lias to the close of the 



Cretaceous. Hardly any forms are known to occur in the 

 Tertiary rocks, but some living species have been detected in 

 the Australian seas. The Triassic genus Myophoria (fig. 355) 

 comprises sub - triangular shells, obliquely keeled, smooth, 

 concentrically striated, or with a partial development of 

 radiating ribs. The left valve has three, and the right two, 

 cardinal teeth. Lastly, Schizodus, ranging from the Upper 

 Silurian to the Permian, comprises trigonal shells in many 



