628 PROCEEDINGS OF THE ACADEMY OF [Oct., 



8 fashion, but only slightly twisted. As in other native American 

 Arionidce, the posterior loop formed by the first and second folds of the 

 intestine lies anterior to that formed by the third and fourth folds. 



The radula has 31.1.33 teeth in two individuals counted. The 

 median tooth is tricuspid, the lateral teeth bicuspid, both being of the 

 type frequently figured for American Arionidce. There are seven 

 laterals on each side (PI. XXVIII, fig. 7). The change to the mar- 

 ginals is gradual, and produced by lengthening the main cusp and dim- 

 inution of the ectocone (PI. XXVIII, fig. 8). The division of the 

 lateral and marginal fields of the radula (between the seventh and 

 eighth teeth) is obvious in a general view of the ribbon under a low 

 power. The marginal teeth (PI. XXVIII, figs. 9, 10, 11) have the 

 basal plates shortened, a single long, lance-shaped cusp springing 

 therefrom. These cusps are rather broad on the inner (fig. 9) and the 

 outer (fig. 11) teeth, but long and graceful in the middle of the marginal 

 field (fig. 10). 



The central nervous system (PL XXVIII, fig. 1) has the commissure 

 between the cerebral ganglia long, even for an Arionid slug. These 

 ganglia are obscurely bilobed. The cerebro-pedal and cerebro-visceral 

 connectives are, on the other hand, excessively short. The sub- 

 oesophageal group of ganglia exhibits stong concentration, with the 

 exception of the pedal mass. 



Affinities of Zacoleus. 



The contiguous posterior insertions of the free retractor muscles 

 and the long cusps of the marginal teeth show that this genus belongs 

 to the subfamily Ariolimacince as defined by Pilsbry and Vanatta.^ 

 The tripartite sole, peculiar genitalia, total absence of ectocones on 

 the Zonitoid marginal teeth, and the extreme shortening of the side 

 connectives of the circumoesophageal ring, all distinguish this genus 

 from its allies. The tripartite sole it has in common with Binneya and 

 Anadenulus, both of which differ conspicuously in more important 

 features. Perhaps the tripartite sole was a character of the primitive 

 Aulacopoda. It seems to occur somewhat sporadically in some genera 

 of several widely separated families of that group. There is no indi- 

 cation of the auto-urotomy (to coin a word) of Prophysaon m our 

 new genus, Zacoleus thus stands isolated among known genera. 

 Notwithstanding its strictly aculeate type of marginal teeth, Zacoleus 



3 Pilsbry and Vanatta, Proc. Acad. Nat. Sci. Phila., 1898, p. 227; PUsbry, 

 Proc. Malac. Soc. London, III, p. 100. 



