MYTILACEA. 



711 



represented in formations as ancient as the Devonian and Car- 

 boniferous, but most of the fossil species are Secondary and Ter- 

 tiary, and there are also numerous living forms. The Recent and 

 Tertiary genus Septifer, again, differs from Mytilus in having a cal- 

 careous shelf within the beak for the attachment of the anterior 

 adductor muscle. The " Date-shells " (Lithodomus) are nearly re- 

 lated to Mod/o/a, and are distinguished by their long, cylindrical, 

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

 in rocks, in which they live. They appear to date from the Car- 

 boniferous rocks, and are known to paleontologists by both their 

 shells and their burrows. Crene//a, of the Cretaceous, Tertiary, and 

 Recent deposits, is another ally of Modiola. 



The genus Dreisse?ia (including Co?igerid) comprises Mussel- 

 shaped Bivalves, with terminal beaks (fig. 581), 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. The hinge is toothed, and the anterior adductor is 

 attached (as in Septife?') to a calcareous shelf within the beak. 

 The living species of Dreissena are inhabitants of fresh or brackish 



Fig. 581. — Dreissena (Cougeria) conglobata, from the Upper Miocene of the Vienna basin. 



(After Zittel.) 



waters, and have a very wide distribution. Fossil forms of the 

 genus are found, often in extraordinary profusion, in the Miocene 

 and Pliocene deposits of Central and Eastern Europe, certain of 

 the Tertiary beds of Austria and Hungary being, for this reason, 

 spoken of as the " Congerien-schichten." The Miocene genus 

 Dreissenomya is related in most respects to Dreisse?ia, but exhibits 

 the remarkable feature that the pallial line is sinuated. 



It is, lastly, probable that the thin-shelled Carboniferous Bivalves 

 which constitute the genus Anthracoptera are brackish-water forms, 

 and are allied to Dreisse?ia. In general form Aiithracoptera re- 



