242 PROCEEDINGS OF THE ACADEMY OF [1898. 



ward from the diaphragm edge, (2) the complex twisting of the gut 

 and the posterior position of the stomach (see these Proceedings, 1896, 

 pi. 13, f. 3) ; (3) the simpler form of the epiphallus, and well developed 

 retractor muscles of vagina and spermatheca, and (4) the normally 

 constructed tail. There are various other minor differences ; but upon 

 the whole, the American genus is less divergent than the European 

 from the primitive forms of the family in its musculature, more diver- 

 gent in the tail structure and epiphallus. In Arion, shell reduction 

 has progressed rather further, at least in the typical forms. Geoma- 

 lacus, a lateral branch from the Avion stem, is even more remote from 

 the American genus. The Himalayan genus Anadenus (with which 

 Fischer unites Prophysaon as a subgenus), differs conspicuously in 

 possessing a well developed penis with retractor, and externally in 

 the extreme narrowness of the foot margin and the tripartite sole. 



The self-amputation of the tail in Prophysaon has been described 

 by Henry Hemphill 13 and W. J. Raymond ; u while Simroth has 

 noticed and figured the appearance of the tail in alcoholic specimens 

 before amputation. 



Mr. Raymond writes as follows: " In August, 1888, I collected 

 on one occasion about a dozen examples of Prophysaon Andersoni 

 J. G. Cp., near the San Jose reservoir, above Lexington, Santa 

 Clara County. While taking measurements of the living specimens, 

 before putting them into alcohol, I noticed in several a contraction 

 about two-thirds of the length from the head. This appeared as an 

 indented line completely encircling the body. Upon handling the 

 slugs to examine this phenomenon more closely, the line became 

 deeper and in the case of two of the specimens the tail dropped off, 

 almost as readily as the ray of the so-called 'brittle' starfish." 



Nearly all full grown alcoholic specimens of P. Andersoni, folio- 

 latum and cceruleum show a well defined impressed line around the 

 tail, or occasionally the tail has actually been amputated, as in the 

 specimen of the last-named species figured on Plate IX, figs. 7 to 11. 

 Dissection shows that the body cavity does not extend beyond the 

 point of excision, or but little beyond ; the remainder of the tail 

 being occupied by very spongy vesicular connective tissue. 



We have not seen Prophysaon alive, and observations are still 

 lacking upon some interesting points which could probably be sup- 



13 Nautilus III, p. 126, footnote; Fourth Supplement to Terr. Moll. V, p. 

 1S2 

 14 Nautilus IV, p. 7. 



