238 PUNCTURELLA. 



septum triangular, very small and short, inclined in about the same 

 plane as the anterior wall of the shell, in the middle of its lower 

 edge produced and thickened like a little short tongue ; about half 

 way between the base of the septum and the outer upper surface of 

 the perforation inside the tube, and at about equal distances from 

 each other and from the median line of the septum, are two little 

 shelly triangular projections which give to the interior of the apex, 

 when looked through, the trilobate outline referred to in the specific 

 name ; base of the shell ovate, the margins showing projections and 

 indentations corresponding to the sculpture of the exterior. Lon. 

 14-0, lat. 10-5, alt. 7-0 mill. (Ball.) 



Yxicatan Strait, 640 fins. 



P. trifolium Dale, Bull. M. C. Z. ix, p. 76, 1881 ; Blake Gastrop. 

 p. 403, t. 26, f. 8, 8b. 



P. eritmeta Verrill. PL 27, figs. 60, 61. 



Shell small, thin, delicate, translucent white, glossy, moderately 

 elevated, with the base between elliptical and ovate, somewhat nar- 

 rowed anteriorly, having both ends evenly rounded and the sides 

 somewhat compressed, but still moderately convex. The apex is 

 minute, nearly central, compressed, turned backward, but scarcely 

 incurved, and with the extreme tip smooth and glossy. The pore is 

 very small, situated very close to the apex, and it appears to be 

 divided by a slight transverse septum, across the middle. The 

 sculpture consists of very numerous radiating striae, which are 

 decussated by fine and close, raised regularly concentric lines of 

 growth of about the same size as the radii on the upper portion, 

 where the shell is minutely cancellated, but on the lower part of the 

 shell the concentric lines become larger and more distant, aud have 

 the form of regular raised cinguli ; the intervals between these, 

 which are two or three times as wide as the ridges, are crossed by 

 the much finer and closer radiating lines, which do not produce a 

 regular cancellated appearance on this part. Both the radiating 

 and longitudinal lines are so fine as scarcely to be visible without a 

 lens. Internally the surface is nearly smooth and lustrous, and the 

 external sculpture shows through the substance of the shell. In 

 the apex there is a minute transverse lamina, forming a small fiat- 

 tend tube. The anterior slope of the shell, seen in profile, is broadly 

 rounded ; the posterior slope falls off abruptly at first, near the apex, 

 and then slopes regularly to the posterior margin, with a nearly 



