230 MOLLUSCA. 



given to the Unios}. The species of this genus appear to 

 commence in the Lower Cretaceous Rocks, and they are 

 very abundant at the present day. The beaks of fossil speci- 

 mens are often deeply eroded, as are those of living forms. 



The Anodons or Swan-mussels closely resemble the Unios, 

 but the shell is edentulous. The earliest fossil forms occur in 

 the Lower Tertiaries (Eocene). 



SECTION B. SIPHONIDA. 



Sub-division I. Integropallialia. Siphons short, pallial line 

 simple. 



FAM. 7. CHAMID^E. Shell inequivalve, attached. Hinge- 

 teeth 2-1 (two in one valve and one in the other). Impres- 

 sions of the adductors large. Mantle closed ; pedal and 

 siphonal orifices small and nearly equal. Foot very small. 

 The most important fossil forms, of this family belong to the 

 genera Chama, Diceras, and Requienia. 



In the genus C/iama the shell is attached usually by the 

 beak of the left valve, but sometimes by that of the right. The 

 upper valve is the smallest, and both bear foliaceous expan- 

 sions. The free valve carries one tooth which articulates with 

 two teeth in the attached valve. The Chamas do not appear 

 as fossils till we reach the Cretaceous Rocks, and they have 

 continued to exist up to the present day. 



In the remarkable genus Diceras (fig. 189), the shell is 



"sub-equivalve, attached by 

 either umbo; beaks very promi- 

 nent, spiral, furrowed externally 

 by ligamental grooves ; hinge 

 very thick, teeth 2-1, promi- 

 nent muscular impressions 

 bounded by long spiral ridges, 

 sometimes obsolete " (Wood- 

 ward). The species of Diceras 

 are exclusively confined to the 

 Middle Oolites. In this for- 

 mation in the Alps they occur 



Fig. ^.-Dicera^arietina. Middle j n such abundance ^ to give 



rise to the name of " Calcaire 



a Dicerates," applied to beds of the same age as the Coral 

 Rag of Britain. 



The genus Requienia is exclusively confined to the Cre- 

 taceous period, and differs from Diceras chiefly in having a 



