GASTROCHiENA. 133 



quite terminal, as there exists a very short front dorsal eclge, 

 about equal to one-sixth of the hinder dorsal line as mea- 

 sured from the beaks to the extreme termination of the 

 shell. The edges of this short dorsal line, which is scarcely- 

 convex and but very slightly declining, are reflected, and 

 in consequence the small surrounding region appears some- 

 what excavated. The hinder dorsal margin, which for a 

 considerable distance is straightish and not at all sloping 

 but rather ascending, finally forming one line with the 

 posterior, sweeps downward with a convex curve, attenu- 

 ating the rounded extremity of the hinder side. The 

 ventral margin, just by its anterior termination, is rounded, 

 and very obliquely ascends in a scarcely convex line towards 

 the front dorsal, by its juncture with which the anterior 

 end is rather sharply angulated. The ligament is rufous, 

 rather long, and slightly prominent : there is not the 

 slightest indication of an umbonal ridge. The hinge con- 

 sists of a not peculiarly small, somewhat spoon-shaped 

 lamina, which projects inwards at some little distance from 

 the anterior extremity. 



The valves rarely exceed three quarters of an inch in 

 length, and about half that measurement in breadth. 



The valves are entirely concealed in a bottle-shaped sheath, 

 of which the bulb is usually an excavation lined with shelly 

 matter, and the neck which projects from the imbedded 

 mass, a bipartite tube, resembling two cylinders laterally 

 fastened together with their touching edges filed away. 

 Authors do not appear to have universally noticed the 

 existence of this envelope, which, however curious in its 

 structure, is certainly a generic and not a specific charac- 

 teristic, since we possess other GastrochatKE of exotic origin, 

 which, although perfectly distinct in the form and character 

 of their valves, have their protecting cells of precisely similar 



