KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 50. N:0 5. 71 



separate species. In V. veliitina, undata. insculpta and lanigera it forms a well-marked 

 börder all aroiind tlie body, in V. plicatilis it is an extremely fine rib. It is a remarkable 

 fact, tliat the fold in tlie Siberian form is distinct all aroiind the body (except perhaps in 

 the hindermost part, where several rugations appear), while in the Gullmar form it be- 

 comes inconspicuous, at least on the right side, and almost disappears outside the pos- 

 terior part of the right muscle. 



In the secondary mantle-border V. undata exhibits a good character; here it is 

 dilated över the columellar portion of the shell as a thin flap; in the other species it is 

 thickened all roiind. 



The ospkradium is in most cases extended in the longitudinal direction, its length being 

 aboiit, or rather more than,twice its breadth. This seems to be the primaryconditionand 

 is found regularly in V. velutina, insculpta (Pl. 2, Fig. 16), lanigera and plicatilis (the Gull- 

 mar form). In the last-named species it has a larger shape in forms with a broader and 

 shorter aperture; I found it so also in the Siberian specimen examined. V. undata ex- 

 hibits the greatest variation; here it may be, though rarely, of a considerable breadth 

 (Pl. 2, Fig. 15). 



The joot has broadly rounded anterior corners in V. velutina and lanigera; in V. 

 plicatilis they become bluntly acuminated and in V. undata and insculpta the corners 

 are acutely produced and directed posteriorly. 



The genital apparatus. This is, as Avas first pointed out by Bergh(1886), of aherma- 

 phrodite type. Bergh gives no detailed description, and done so since no one has. 

 The following accounts of V. velutina and V. plicatilis are based on a microscopical 

 examination of a series of sections, that of V. undata is based on dissections. 



In V. velutina, Gullmarn (Text Fig. 2), the female genital orifice (^) leads on one 

 side into a short but broad vagina, on the other into a wide bursa copulatrix {h. c). The 

 former extends backwards and receives, at its innermost, somewhat bulbiform end, the 

 hermaphrodite duct [h. d.). This in its distal part is straight, but becomes dicotomi- 

 cally branched backwards and filled with spermatozoids; the uppermost ends of the 

 branches are ovarial follicles. The construction of the hermaphrodite gland thus cor- 

 responds to that of Marsenina and Onchidiopsis. 



As already stated, the bursa copulatrix communicates in its före part with the 

 end of vagina. It covers the upper side of the latter like a curved säck and at its posterior 

 end it opens again into the postero-lateral portion of the vaginal säck. 



The male genital apparatus consists of a broad disk-like projecting penis Avith a 

 veiy minute tongue-shaped process, containing the aperture of vas deferens, adhering 

 to its outer and lower margin (cfr. Bergh 1887, Taf. M, Fig. 20). Vas deferens {v. d.) is a 

 narrow canal of a weak and soft structure embedded in the muscular wall of the body; it 

 stretches backwards to the region outside the female orifice, on the upper side of theshell- 

 muscle. Here it receives a duct emerging from a posterior gland {pr.), then continues some- 

 what backwards and debouches into a side portion of the vaginal säck. The gland just 

 named has the same histological structure as the prostata gland in Marsenina (quod 

 vide), with cystoid lobuli, lined with a high excretory epithelium containing big concre- 

 tions. This prostata gland, in Marsenina debouching directly into the vagina, has con- 



