124 BULLETIN OF THE 



seem to me that they can be separated, except in a merely sectional sense. But 

 so capricious are the characters, that, until each species has been thoroughly 

 examined as to its anatomical relations, it will not be possible finally to 

 determine its systematic place. 



It is just as well here to call attention to a peculiarity which is liable to 

 deceive students. The nuclear or larval shells of the Pleurotomidce may be 

 broadly classified in two groups. In one the shell is horny, dark colored, and 

 in a very large number of cases has the pretty oblique reticulation of curved 

 lines which distinguish what I have called, after Orbigny, the Sinusigera 

 nucleus. In the other group the larval shell is glassy or porcellanous, much 

 like the rest of the shell, except that it is usually more translucent. Both 

 kinds of nucleus have a very similar series of subordinate modifications, of 

 which one might construct a whole systematic classification. Neither sort of 

 larval shell is confined to this family. 



Now, after the larva has assumed the adolescent features of the species, the 

 thin horny kind of nucleus is a weak point, easily abraded or drilled, and liable 

 to decay. So, at an early stage, the animal begins to deposit a shelly layer 

 inside of the horny one and closely imitating its coils, but of course smoothly 

 rounded like the inside of the original shell, and always destitute of the exter- 

 nal Sinusigera striation or other sculpture. It often happens that the shell is 

 filled solidly with the new deposit, and that the horny envelope decays, leaving 

 no trace, while there will be no sign of erosion, and the internal deposit will 

 have all the effect of being an original nucleus of the shelly sort. This being 

 accomplished, it only needs a few of the superficial variations of sculpture and 

 relative slenderness to make very fair grounds for the description of a new 

 species. "It has such and such differences," the observer will reason, "but 

 above all the nucleus is totally distinct." As the writer has been several times 

 nearly taken in in this way, he thinks a warning may be useful for others. 



? Pleurotomella catasarca n. s. 



Shell small, fusiform, compact, with a turbinate Sinusigera nucleus of three 

 whorls and seven succeeding whorls, a keeled periphery and no ribs; exterior 

 pale straw-color with darker axially directed cloudings, following the incre- 

 mental lines ; spiral sculpture of a well-marked but not sharp-edged keel at 

 the periphery, which on the earlier whorls is minutely undulated; the fasci- 

 ole extends forward from the suture nearly to the keel, but falls short a little, 

 and is bounded by an obscure, little elevated ridge parallel Vvith the keel, 

 marked by one or two especially prominent fine revolving threads; the whole 

 space between the keel and the suture behind it is covered with these fine 

 threads with slightly wider interspaces, and crossed by delicate arched ripples, 

 neatly and uniformly spaced. In front of the keel the spirals are divisible 

 into primary (10-15) and secondary; the latter, much finer than the others, 

 run in the interspaces in groups of two or three in each interspace, except on 



