STROPHODONTiE OF THE HAMILTON GROUP. 9t 



Area of the ventral valve nearly on a plane with the axis of the shell, 

 ahout a line in width, gently narrowing towards the extremities, ver- 

 tically striated, with the margin crenulated for more than half the 

 distance from the centre to the extremities. Area of the dorsal valve 

 almost rectangular to that of the ventral valve, very narrow and nearly 

 linear throughout, sometimes narrower in the middle, striate and cre- 

 nulate as in the opposite valve ; with a small, smooth triangular space 

 beneath the apex. 



The surface of the ventral valve is marked by sharply elevated, strongly 

 crenulated striae, between which are sometimes one or two less elevar 

 ted striae similarly crenulated, and still finer striae between the latter. 

 In other specimens there are wider spaces of finer equal striae between 

 the stronger ones ; and, in still other examples, the striae are nearly 

 all strong and sharply elevated, with few finer ones, which soon rise 

 to the strength of the others. Close undulating concentric striae cover 

 the whole surface. The dorsal valve is marked by distant sharp ele- 

 vated striae, between which there are from three to six and rarely ten 

 finer striae, which are very finely crenulated by concentric striae. 



In some specimens the ventral valve is marked by an irregular fold or 

 ridge down the middle, and there are sometimes a few incipient plica- 

 tions on one or both sides towards the margin of the shell. These plica- 

 tions likewise affect the dorsal valve. 



The interior of the valves is finely pustulose. The divaricator muscu- 

 lar impressions of the ventral valve are large and spreading, about as 

 wide as long, extending nearly half the length of the valve, and deeply 

 striate ; while the occlusor impressions are elongate-ovate or cordiform, 

 and strongly marked. The muscular impressions of the dorsal valve are 

 strong and divided above by a rounded ridge which supports the strong 

 bifurcate cardinal process, each division of which is bilobed, and the sur- 

 faces roughened for the muscular attachment. 



Some of the more characteristic forme of this species are illustrated on Plate xvi. In a 

 . large number of specimens, the convexity of the ventral valve is less than in the 



figure given. 

 [ PaLvKOntohooy IV.] 18 



