62 PALEONTOLOGY 



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. Pcctunculi appear 

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

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

 genus found in the Belgian eocene has the ligamental area 



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

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

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

 found in the cretaceous and tertiary strata. 



The Trigoniadcc are represented in the lower Silurian 

 strata by Lyrodesma (fig. 13, 6), a shell with several radiating 

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

 by Axinus (fig. 13, 7) and several other imperfectly-known 

 genera. Schizodus occurs in the permian at Garford (with 

 Turbo and Rissoa), near Manchester. The trias contains true 

 Trigonice associated with the genus Myophoria (fig. 14, 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 100 species are known in the secondary rocks, and 

 two are still living on the coasts of South Australia. 



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

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

 distinguishable from recent shells ; but those of the coal 

 measures and older rocks are extremely problematic, and may 

 even belong to marine genera. 



Of the genus Ghama there is one species in the upper 

 greensand and chalk of England, and another in the London 

 clay. Elsewhere they are more abundant, amounting to thirty 

 species. Closely allied to Chama is the Diceras (Lam.), of 

 which the remarkable casts attracted attention at an early 



