394 PROCEEDINGS OF THE ACADEMY OF [1893. 



comparison, that in Limax tenellus Nils., all (32) marginals were 

 found to have a distinct "outer cusp"; but on the marginals of L. 

 dnereus Lister (about 130 on each side) no ectodonts were seen. 



A similar formation of the earliest teeth will probably be found 

 in other Limaces, and in such Zonitidce as Hyalinia radiatula Gray, 

 petrophila Bid., capsella Gld., etc., species in which the laterals bear 

 distinct endodonts. 



Zonitoides arboreus Say. PI. 11, figs. I, II. 



No embryo of this species was obtained, but a very young spec- 

 imen with shell and soft parts nearly colorless, the former measuring 

 1*1 mm. diameter, with IV whorls. The radula (fig. I), was 0*39 

 mm. long, and had 4<S transverse rows, of which, however, some of 

 the youngest, at the posterior end, could not be clearly seen. Here 

 there are 7 - c*7 teeth, three of which are rather well formed laterals. 

 Very few rows and teeth could have been lost from the front end of 

 the radula, and the first present in the specimen examined were evi- 

 dently of primitive shape, and a type entirely different from that 

 of Limax, rather resembling those of Patula striatella, couf. fig. 

 VIII. 



The central teeth have a wide, short base or "body," with a dis- 

 tinct mesodont and at least quite early a side cusp on each side. 

 Comparatively soon the plate of attachment is seen posteriorly, 

 though very small. The laterals (about 5 in the adult) are wide 

 transverse plates, somewhat irregular, longer in the mesial half, with 

 a short, wide, blunt mesodont gradually becoming longer and more 

 slender, and 2 or 3 small cusps in place of the later ectodont. No 

 trace of an endodont is seen. As the teeth without much change 

 gain their definite shape, the plates of attachment also develop. 



Fig. II represents the radula of a nearly adult specimen, the shell 

 having a diameter of 4-3 mm. The radula is 1-05 mm. long, 0*35 

 wide, and has 77 transverse rows of 17*5 c. 5 '17 teeth. Only the 

 first marginal bears an ectodont. Another example, of 4-5 mm. 

 diameter, had a radula 1-33 mm. long, 0-45 wide, with 19-1x5- c. 

 5x1*19 teeth. Another of a peculiar form from Alabama gave the 

 formula 20-1x4- c. -4x1 -20. 



Embryonic specimens remain to be examined, and the earliest 

 form of the central and inner lateral teeth ascertained. 



