ASIPHONIDA. 483 



is equivalve and wedge-shaped ; the beaks are placed quite 

 at the anterior end of the shell, and the posterior end is 

 truncated and gaping. The hinge is toothless, and there is 

 an elongated ligamental groove. The genus commences in 

 the Devonian, is well represented in the Carboniferous as 

 well as in later deposits, and exists under various specific 

 forms at the present day. Trichites, of the Jurassic and 

 Cretaceous, resembles the preceding, but has an inequivalve 

 shell and twisted beaks ; while the Aviculopinna of the Per- 

 mian has the beaks not altogether terminal. 



FAM. 3. MYTILID^E. Shell equivalve; umbones anterior; 

 hinge, typically, edentulous ; anterior muscular impression 

 small, posterior large. Shell attached by a byssus. Mantle- 

 lobes united between the siphonal apertures. Foot cylin- 

 drical, grooved, and byssiferous. 



In the genus Mytilus are the true Mussels, in which the 

 shell is wedge-shaped, and the beaks terminal. Numerous 

 fossil forms are known, commencing in the Permian. The 

 Modiolce, or " H-^rse-mussels," have the beaks anterior, blunt, 

 not pointed, the hinge edentulous, and the shell oblong. 

 More than one hundred fossil species have been described, 

 commencing in the Lias, and extending to the present day. 

 The Palaeozoic Modiolce are probably referable to different 

 genera. The Date-shells (lAikodomus) form a sub-genus of 

 Modiola, and are distinguished by their long, cylindrical, 

 anteriorly-inflated shell, and by their habit of forming per- 

 forations in rocks, in which they live. They appear to date 

 from the Carboniferous rocks, and are known to palaeon- 

 tologists by both their shells and their burrows. Crenella, 

 of the Cretaceous, Tertiary, and Eecent, is another ally of 

 Modiola. 



The genus Dreissena (including Congeria) comprises Mussel- 

 shaped Bivalves, with terminal beaks, and a small byssal notch 

 in the right valve, but differing from Mytilus in having keeled 

 valves, and in the fact that the internal lining of the shell is 

 not nacreous. Fossil species of this genus appear in the 

 Tertiary rocks, in certain parts of which they are present 

 in great abundance. The Carboniferous genus Anthracoptera 

 may be regarded as probably related to Dreissena. 



