BRACHIOPODA. 455 



convex or concave, and the dorsal valve follows the curva- 

 ture of the ventral valve. The species of the genus Stro- 

 pkomena are very abundant in the Silurian, Devonian, and 

 Carboniferous formations, often attaining a large size ; but 

 (except under a peculiar sub-generic type) they do not seem 

 to have survived the close of the last-named period. 



Speaking generally, the species of StropJiomena may be 

 distinguished from those of Orthis and Leptcena both of 

 which they much resemble occasionally by attention to the 

 following points : The shell is usually flat and semi-oval, its 

 length and breadth being about equal, and the hinge-line 

 always equalling and often exceeding in length the transverse 

 diameter of the shell. Earely, the shell is bent and trans- 

 versely extended, as in Leplcena. The cardinal process is 

 large and bifid ; and the muscular impressions are quadrate 

 arid laterally expanded. 



It may be appropriately added in this connection that the 

 name Stropkomena is employed above in the wide general 

 sense in which it has been commonly used by most palaeon- 

 tologists. Certain forms of the genus, however, with twisted 

 beaks to the ventral valve, and often a high false area, are 

 commonly separated to form a distinct section (Hemipronites 

 of Pander, or Streptorhynchus of King); and 

 the name of Stroplwdonta has been given to 

 others in which the hinge-line is crenulated, 

 and there is no fissure in the hinge-area of the 

 ventral valve. 



In the genus Leptcena are forms smaller 

 than the Str&phommat, but resembling them 

 in many respects. The shell is more or less 

 completely semicircular (figs. 306, 307), with teIaLto2eo. 

 a double hinge -area, notched in the centre, 

 the fissure in the ventral valve being partly figure indicates the 



T rm i real Size f the 



covered by a deltidium. The valves articu- shell. 

 late by teeth and sockets, and the surface is 

 generally striated. The Leptccnce extend from the Silurian 

 rocks to the summit of the Lias, but are not known in any 

 younger deposits. Speaking generally, Leptcenee can be sep- 

 arated from Orthis and StropJiomena by the involute, trans- 



