130 INVERTEBRATA. 



Measurements. Length, 33 mm. ; breadth, 2G mm. ; thickness, 

 18 mm. 



Another specimen gives length 30 mm. ; breadth, 23 mm. ; 

 thickness, 20 mm. 



Terebratella Keepingii, Walker. 

 (Plate vii. fig. 19.) 



Shell elongated, subpentagonal, or sometimes trigonal ; pos- 

 terior end produced. Front margin truncated, forming distinct 

 lateral angles. 



The whole shell is covered with a beautiful delicate striation 

 radiating from the beaks, new striae or ribs being frequently inter- 

 calated towards the front. Striae rounded, about equal to the 

 interspaces in size. 



Dorsal valve moderately convex, flattened along the middle 

 and depressed in front. Dorsal septum elongated. 



The ventral valve is convex with steep sides, straight in front, 

 strongly arched across behind, and produced into a very prominent 

 long beak. Beak ridges well defined and sharp near the foramen. 

 Area elongated, slightly convex. Deltidium elongated in two 

 pieces. Foramen small. 



This shell varies considerably : the typical shell is pentagonal 

 in form, but some specimens have the long sac-like shape of T. 

 digona ; they vary also in the length of the shell and coarseness 

 of striation ; the latter is however always much more delicate than 

 in T. Davidsoni. 



The last-named species is the nearest ally of T. Keepingii, and 

 is the only one with which it could be confounded. Our new 

 species is distinguished by its greater length, especially length of 

 beak and area, its steep sides, truncated front, and delicate striae. 

 It is an extremely well-marked type, although as I have shewn 

 elsewhere the passage forms have been found connecting it with 

 T. Davidsoni (see ante, p. 25). 



Measurements. Length, 23 mm. ; greatest breadth, 17 mm. ; 

 thickness, 11 mm. 



