GENERA WHICH LIVE IN FRESH WATER. 



441 



like appendages, acquires a circular outline and a crescent-shaped den- 

 ticulate hinge-line, and becomes the genus Pectunculus. Isoarca is 

 another modification in which the shell grows most rapidly at the 

 anterior margin, so that the beaks recede from each other and become 

 spiral, and the shell is globular. Pecten, by thickening the lower 

 valve at the hinge, gives rise to the genus Spondylus ; and Spondylus, 

 by losing the thickening and the ear-like appendages of the hinge, but 

 retaining the interlocking denticles of the hinge, becomes the genus 

 Plicatula. We might in this way show how all the genera of shells 

 are related to each other, and may well have been derived from a few 

 recognisable types by growth in some directions and atrophy in 

 others. But our object is rather to exhibit the nature of generic 

 characters, so that the eye of the student may be educated to discern 

 the characters which follow a plan, and thus discriminate genera, 1 

 recognise their affinities, and identify species. 



Fresh- water Deposits. The great divisions of strata into fresh- 

 water and marine deposits are recognised by fossils. Some fresh- water 

 strata certainly were deposited during the Primary period, especially 

 among the Coal-Measures ; but with the exception of Anodonta Jukesii, 

 found in the Upper Old Red Sandstone of Kiltorkan, in Kilkenny, 

 they yield no fresh- water fossil referable to an existing genus. But in 

 the Secondary and Tertiary periods, not only are all the fossils which 

 occur in British fresh-water beds species of existing genera, but the 

 genera are all of the same types as still live in fresh waters in the 

 British area. Among GASTEROPODA the characteristic genera are 



Fig. 82. Plunorbis. 

 J'lanorbis. 

 VaLvata. 



Fig. 83 Paludina. 

 Paludinn. 



Limnaa. 



Physa. Valvata. Neritina. 



The characteristic LAMELLIBRANCHIATA are 



A ncylus. 



Unio. 



Fig. 84. Cyrena. 

 Anodonta. 

 Cydas. 



" Identification of Fossil Bivalve Shells," Geologist, Feb. 1864. 

 2 Several marine types have fresh-water representatives. Thus the marine 

 Mytilus is represented by Dreissena, the marine A rat by fresh-water species in 

 the Ganges and South-east Asia. 



