396 PROCEEDINGS OF THE ACADEMY OF [1893. 



in this as well as some other species. The first transverse row is 

 represented only by the first lateral on the right ; the second by 1 'c - l 

 and so on, as shown in the figure. The transformation is rather 

 simple and decidedly slow. The centrals are wide and short, at first 

 with a short, wide, blunt median projection which develops into the 

 mesodont, and very slowly, like the side cusps. All other teeth are 

 first formed as simple transverse bars or plates, the laterals gradually 

 assuming their definite shapes without intermediate stages ; even the 

 ectodonts seem to be always single. Otherwise there is much sim- 

 ilarity to Z. arboreus, but the forms are somewhat more plump. 



The first formation and development of the marginals has not yet 

 been observed with sufficient exactness. The mesodont and partly 

 the ectodont are "double- pointed," and it appears probable that the 

 mesial small cusp of the former is really the entodont, as it first 

 stands near the base (fig. VII, 15) and then gradually ascends on 

 the mesodont and becomes evanescent, as seen in fig. VII, 12-8. 

 This figure represents the radula, 1 4 mm. long, from a specimen hav- 

 ing a shell of 5-8 mm. diameter, apparently not quite mature. Yet in 

 all the 94 transverse rows, no new teeth are added ; the formula is 

 c. 6x1:10. There is very little change in the shapes of the siugle 

 teeth, except that the plates of the sixth and seventh laterals are 

 noticeably longer in the posterior than in the front part of the radula. 



Of Patula alternata Say, a number of examples were examined. 

 One having a shell of 20 mm. diameter had a radula 4-5 mm. long, 

 1-5 wide, with 143 transverse rows of 34-C-34 teeth, with about 15 

 laterals. In a young specimen of 3 mm. diameter the radula was 0*8<S 

 mm. long, and had 72 transverse rows of 11-c-ll in the anterior, 

 12-cT2 in the posterior part. 



The radula of Patula solitaria Say, extracted from a shell of 23 

 mm. diameter, is 5 mm. long, 1 -8 wide, with 142 transverse rows of 

 31'c31 teeth, 16 laterals, In a young one, the shell of 5-3 mm. 

 diameter, I counted 68 transverse rows of 13 , c , 13 teeth in the 

 anterior 15'C*15 in the posterior part. 



Polygyra* (Triodopsis) tridentata Say. PI. 11, figs. V. VI. 



No embryo or quite young specimens were at my disposition. 

 The smallest found had a shell measuring about 6 mm. in diameter. 



"Sec Pilsbry, Preliminary outline of a new classification of the Helices. 

 Proc. Acad. Nat. Sc, Phila., 1892, p. 400. 



