LAMELLIBEANCHIATA. 71 



Others, with prominent unibones, teeth like Nucula, and a 

 striated ligamental area, form the genus lsoarca of Minister 

 (fig. 18, 7). Above 200 species of Nucula and Leda are 

 known only as fossils, and range through all the rock systems. 

 The palaeozoic species are anomalous in form, and when better 

 understood, will certainly be considered distinct as genera. 

 Yoldia is a newer tertiary form characteristic of high northern 

 latitudes ; and Solenella occurs fossil in Patagonia and New 

 Zealand. The problematic genus Solemya is supposed to 

 have existed in the carboniferous period. Pectuncidi appeal- 

 first in the cretaceous strata, being less ancient than Limopsis, 

 which occurs in the Bath oolite. A member of the latter 

 genus found in the Belgian eocene has the ligamental area 

 entirely behind the cartilage-pit, and is called Nucunella by 

 d'Orbigny. The " Stalagmium" of Conrad (=Myopara, Lea) 

 is identical with Crenella (T. Br.), a sub-genus of Modiola, 

 found in the cretaceous and tertiary strata. 



The Trigoniadce are represented in the lower Silurian 

 strata by Lyrodesma (fig. 17, 6), a shell with several radiating 

 hinge-teeth, striated transversely ; and in the upper palaeozoics 

 by Axinus (fig. 17, 7) and several other imperfectly-known 

 genera. Axinus occurs in the magnesian limestone of Durham, 

 and in the permian (with Turbo and Rissoa) at Garford, near 

 Manchester. The trias contains true Trigonice associated 

 with the genus MyopTwria (fig. 18, 8), which has the umbones 

 turned forwards, and a posterior hinge -tooth. The only 

 member of this family which has yet been found in tertiary 

 strata is the little genus Verticordia (Wood) of the crag. No 

 Trigonice have been met with, although 1 00 species are known 

 in the secondary rocks, and two are still living on the coasts 

 of South Australia. 



Fresh-water mussels (JJnionidce), of large size and various 

 form, occur in the Wealden formation, and are not generically 

 distinguishable from recent shells; but those of the coal 



