THRACIA. 225 



variable ; the front extremity is rounded, but rarely sym- 

 metrically so, and is almost always attenuated and narrower 

 than the hinder termination ; this latter is bluntly biangu- 

 lated, with the upper edge in typical examples projecting 

 beyond the lower, so that a perpendicular line drawn from 

 the dorsal end would fall beyond the ventral margin. The 

 umbonal ridge is distinctly indicated, is not very oblique, 

 and has a rather large area behind it. The beaks project 

 moderately, and do not lean to either side ; the ligament 

 is rather large and prominent, elongated, and of a buff 

 yellow. The cardinal plate which supports the ligament is 

 very narrow, peculiarly trigonal, and inclining outwards. 

 The ossicle is solid, longer than in pJiaseoUna, and almost 

 linear, resembling a portion of a broken ring. 



The preceding species, although very nearly allied to the 

 present, has a more elongated and oblique contour, a more 

 delicate texture, and a smoother surface. Its dorsal edges 

 are less sloping; the front one, which is convex or even 

 arcuated, scarcely declines at all, so that the anterior side 

 is much the broader ; the ventral edge too both rises more 

 behind and swells out more in front. The beaks are more 

 inclined ; the ligament smaller and darker ; and the ossicle 

 is filled up, as it were, in the middle by shelly matter. In 

 the more typical examples of phaseoUna, the hinder termi- 

 nation is usually coated with a rust-like substance, and 

 projects below as much at least as it does above ; its 

 dorsal edge inclines upward. The cardinal plate is rather 

 stronger, and produced more under the umbo, making 

 the sinus more oblique ; the muscular impression beneath 

 the beaks is rather broader, and the sinus of the pal leal 

 impression more profound and running more to a point 

 below. 



The superficial dimensions are almost the same as in the 



VOL. I. G G 



