INSTITUTE OF SCIENCE, PHILADELPHIA. 317 



The appearance of the figures given on plate i6 will doubtless arouse 

 some skepticism in regard to including them in a single species. I should be 

 skeptical myself if I had only the figures to judge by; but I must remind my 

 readers that the figures represent only extremes ; the gradations between 

 which are extant and may be examined in the collection, but which I could 

 hardly try to figure in extenso. 



A fact which I have verified in my study of the genus Turritella is that 

 in many forms the rotundity or flatness of the whorls is a feature which 

 marches with elevated spiral sculpture even when the rotundity would remain 

 if the spirals were ground off the whorl. And this is not merely true of dif- 

 ferent specimens, but may be verified by the student on one and the same speci- 

 men when, as sometimes happens, the early whorls are destitute of spirals. It 

 will be seen that, in such a case, the whorls near the apex are much flatter 

 than those later on where the elevated spiral sculpture is developed. Of 

 course, there may be species without spiral sculpture which have round whorls, 

 or species having spiral elevated lines on flat whorls, but that does not invali- 

 date the observation as above limited. 



It will be observed that the flat-sided specimens, figures ii and 12, have 

 little or no elevated spiral sculpture, whereas it is strong on figures 10 and 

 13, which have round whorls. 



1. Variety mediosidcata Heilprin, pi. 16, fig. 12. Beginning with the 

 most feebly sculptured form, we find that it has fine spiral striae all over the 

 surface, there is a feeble constriction of the equator of the whorl, in front of 

 which are three faintly indicated raised spirals, and behind which are two 

 still fainter ones, slightly rippled by the incremental lines. 



This variety is very variable, no two specimens showing exactly the 

 same sculpture ; the specimen figured is Prof. Heilprin's type. Most of 

 the specimens have slightly rounder whorls and stronger primary spirals than 

 this one, but the variety is a rare one compared with the typical form. 



2. Variety cingidata Heilprin, pi. 16, fig. 11. Here we have the equator 

 with three narrow sulci instead of one broad one, and a faint spiral rib in front 

 of these. The surface, where not worn, is covered with fine spiral striae. The 

 whorls are rather flat, and the variety is very rare. I have seen but two speci- 

 mens. It leads directly to the next variety. 



3. Variety tensa Dall, pi. 16, fig. 13. In this form we have first the faint 

 anterior three and posterior two faint spirals of mediosidcata become strong, 

 their interspaces narrow and deeper, and the equatorial sulcus also narrower. 

 The spirals are rippled by the incremental lines and the whorl is slightly 

 more rounded ; the fine striae persist over the surface. An absolutely un- 

 broken transition can be traced between the specimens figured at numbers 11 

 and 13. The type measures 78 by 13 mm. and came from the Caloosahatchie, 

 where this variety is rare, but it is very common on Shell Creek. 



4. T. apicalis, typical form, pi, 16, fig. 10. In this we have a beaded 



