150 BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



General appearance. Shell conical, somewhat convex ; white, sometimes 

 tinted yellowish, from the thin investing membrane ; the produced tips of the 

 terga are purple ; the parietes of each compartment have from two to four 

 strong, prominent, sharp, straight, longitudinal ribs ; these are sometimes irreg- 

 ular, and rarely, as will presently be described, they are absent. The radii are 

 smooth and of considerable breadth ; their summits are nearly parallel to the 

 basis or only slightly oblique ; hence the orifice is entire ; it is rather small and 

 ovate, being broad at the rostral end and very sharp and narrow at the carinal 

 end. 



The scutum is more or less concave from end to end, with sculpture 

 of narrow, prominent growth-ridges, crossed by fine longitudinal 

 strice which crenulate the ridges. The articular ridge is low, rather 

 abruptly terminated below. Usually there is no distinct adductor 

 ridge, but the callous occupying its place has a slightly raised, acute 

 border, bounding the very deep pit for the lateral depressor muscle, 

 and the border of the adductor muscle pit is slightly prominent and 

 angular. Sometimes these prominences stand in line and are 

 weakly connected, forming thus a rather ill-defined adductor ridge. 



The tergum is very much narrower than the scutum, its length 

 nearly twice the width. Its pointed beak, of a purplish or pale-pink 

 tint, or sometimes white, projects wett above the scutum. Exter- 

 nally it is marked with somewhat coarse growth-lines. There is no 

 longitudinal furrow, but a wide spur-fasciole is defined by impressed 

 lines. Spur close to the basiscutal angle, truncate distally, the basal 

 margin sloping to it on both sides. The articular ridge is rather 

 low; scutal margin is sharply inflexed in the low T er part. Crests 

 for the depressor muscles are irregular, not very strong. 



Darwin says: 



The parietes have large, square parietal tubes ; in the upper part these 

 are filled up solidly without transverse septa; the longitudinal septa are finely 

 denticulated at their bases, and the denticuli extend unusually close to the outer 

 lamina. In very young specimens the inner lamina of the parietes is ribbed in 

 lines corresponding with the longitudinal septa, as is the case with most species 

 of the genus; but in medium and large-sized specimens there are between the 

 ribs, thus produced, from one to four smaller ribs, which do not correspond with 

 any longitudinal septa ; they are finely denticulated at their bases and may be 

 considered as the representatives of longitudinal septa which have not been 

 developed and reached the outer lamina. I have seen no other instance of this 

 structure, namely, the presence of a greater number of ribs, on the inner lamina 

 of the walls, than there are longitudinal septa. [B. rostratus is similar in this 

 respect.] The radii have their summits generally parallel to the surface of 

 attachment, as is usual in the first section of the genus, but sometimes they are 

 slightly oblique; the septa sometimes rudely branch a little, but they exhibit 

 scarcely a trace of denticuli ; the interspaces are filled up quite solidly. The 

 alse have their summits very oblique ; their sutural edges are finely crenated. 



Basis rather thin, translucent, not permeated by pores ; obscurely furrowed 

 in lines radiating from the center ; the circumference is marked in a peculiar 

 manner by the longitudinal septa and by the tips of those intermediate den- 

 ticulated ribs which occur on the inner lamina of the parietes. 



