1893.] NATURAL SCIENCES OF PHILADELPHIA. 279 



somewhat angular at the periphery ; the shells show distinct ribs 

 and the membranous " ribs " upon them are quite small and fine, 

 while in costata the ribs are quite small ; the membranes are as a 

 rule, coarse. In the former, the ribs are also more numerous, and 

 more oblique, especially as seen from above. The lip, in fresh 

 specimens, is pure white, never so in costata, and of a somewhat 

 different form. 



As said above, the specimens seen from the states west of the 

 Mississippi, must be referred to V. gracilicosta instead of to costata. 



EXPLANATION OF PLATE VIII. 



(Plates 32 and 33 referred to throughout the text will be found in 

 Pilsbry's continuation of Tryon's Manual of Conchology (2), VIII. 



Fig. A-F Radula, enlarged about 900 diam. The first laterals and 

 last marginals are always represented, so that the last 

 gives the number in the half transverse row. 

 Radula of V. pidchella ; 3 is nearly like 4 in B. 

 Radula of V. excentrica. 

 Radula of V. costata. 

 Radula of V. albula. 

 Radula of V. cyclop horella. 

 Radula of V. perspectiva. 



Radula of V. pidchella, the central and first lateral in the 

 first (anterior) transverse rows, with cusps worn off. 

 Jaw of V. pidchella as seen under moderate enlargement, 

 cutting-edge below, as in all figs: scale about 150 : 1. 



Fig. I. The same, another specimen, enlarged nearly 300 diam. ; 

 a b the posterior plate, extending backward into the tena- 

 culum membrane t t. 



Fig. K. The same, another specimen, as seen in profile. 



Fig. L. The same, median section (partially diagrammatic). 



Fig. M. Jaw of V. excentrica. 



Fig. N. Jaw of V. costata. 



Fig. O. Jaw of V. albida, extended. 



Fig. P. Central tooth of the radula (of V. parvula), half profile 

 view. 



Fig. R. One-half jaw of V. parvula, seen obliquely from above. 



