222 PROCEEDINGS OF THE ACADEMY OF [1898. 



gradually into the marginal series by shortening of the basal-plates. 

 Marginals bicuspid, with occasionally a tooth with the ectocone 

 bifid. In the Ariolimacince the inner cusps of the outer lateral and 

 inner marginal teeth are much lengthened and oblique, the ectocones 

 much reduced, simulating the pseudo-zonitoid teeth of Flammulina. 

 Arion also shows this tendency to a much less degree. In the other 

 genera the cusps of these teeth are short or of moderate length. 



All ArionidaB have four longitudinal folds of the intestine, 5 as 

 usual in slugs generally ; the main divergence being in the compar- 

 ative length of the folds and the degree of their torsion. The folds 

 are designated by the initial G with exponents 1, 2, 3, 4, as in PI. 

 XI, fig. 29. The anterior loop between G 2 and G 3 is caught up by 

 the cephalic artery immediately upon its emergence from the di- 

 aphragm (PI. XIV, figs. 66, 70, 72). In Arion the posterior loop 

 between G 1 and G 2 , forming the lower end of the stomach, lies pos- 

 terior to all other intestine folds; 6 but in all the other American 

 genera the loop formed by G 3 and G 4 lies behind the stomach. 



In Ariolimax, Aphallarion and Hespe> arion (PI. XI, fig. 33) the 

 gut is long and spirally wound. In Prophysaon (PL XI, figs. 28- 

 30, 32, 34), Anadenulus (fig. 35), Hemphillia (fig. 36) and Binneya 

 (fig. 31) it is much less twisted spirally. In the latter two genera 

 the posterior loop formed by G 3 and G 4 is very short, on account of 

 the crowding forward of the viscera into the dorsal hump. 



In Prouhysaon, Anadenulus, Hemphillia and Binneya there is no 

 differentiation of the first fold into crop and stomach, such as occurs 

 in Ariolimax, etc. 



Reproductive organs. — In the Arionidoz there are two main types 

 of genital organs. In the more primitive and normal type there is 

 a well developed penis provided with a retractor muscle, and in 

 every way homologous with the same organ in the Helices and pul- 

 monate snails generally. This may be seen in Ariolimax, Hesper- 

 arion (PL XII, figs. 44, 47), Binneya, Hemphillia, and the Himal- 

 ayan genus Anadenus. In the slugs, as well as in many other 

 snails, the terminal portion of the vas deferens is noticeably enlarged, 

 often considerably swollen for a distance above its insertion in the 

 penis (see PI. XII, fig. 49, epi., Hemphillia ; and these Proceedings, 

 1896, PL XIV, fig. 14, epi., Ariolimax). This enlarged tract has 



5 In Aphallarion there is a short loop interposed between the second and 

 third long folds, making six folds. See P. A. N. S., 1S96, pi. 13, f. 4. 



6 P. A. N. S., 1896, PI. 13, f. 3. 



