80 NILS HJ. ODHNER, MOLLUSCA. 
no obvious furrows in the umbilicus, which are, however, indistinctly indicated. M. 
kreffti is more elevated and more or less distinctly spirally striated, its height 1 */s 
of the aperture breadth, and larger, 11.5 mm in diameter at 5 whorls. M. fulva is 
smooth, smaller, 8 mm at 5 whorls, and its height is little more than the breadth 
of the aperture. 
All the species in question agree in the following exterior characteristics of the 
animal. The foot sole is divided all along into a median and two lateral areas. 
The foot end has a slightly elevated upper corner but no horn. On the sides of the 
foot there run two epipodial furrows parallel to each other. 
Between the different species differences prevail in colour and in the form of 
the shell lobes. In M. fulva the foot, on its dorsal side, is of a blackish blue colour; 
likewise the shell lobes, which are of nearly uniform lanceolate form; in M. respon- 
sivus the foot has a grayish blue back and the right shell lobe is grayish in its 
middle (and larger than the left one). M. marmorata and rustica are light coloured 
on foot and shell lobes, except that the right shell lobe of the former has a blackish 
base; in both the right lobe is the larger. Of the cervical lobes the left one is always 
Fig. 30. Jaws of Microcystis marmorata (a), M. fulva (b), M. rustica (c), M. responsivus (d). 
smaller than the right, which is divided into a columellar and a median portion; 
they are of the same colour as the shell lobes. 
The genital apparatus exhibits the following common conditions: From the pro- 
stata gland, which is spread over the inside of the uterus, a simple, winding or straight, 
vas deferens leads to the penis, passing the right tentacular muscle. It has no acces- 
sory coeca. Above the insertion point of the vas deferens the penis carries a more 
(M. fulva, fig. 28) or less (M. responsivus, rustica, fig. 29) produced flagellum. In the 
first case the vas deferens debouches in a sort of epiphallus (ep.). At the end of 
the oviduct is fixed a lengthened coecal sac, containing in M. fulva a chitinous cord 
corresponding to the dart of the Helicidae and certain Naninidae; this dart sac seems 
to be of varying length in the different species. 
The jaw has always a single median tooth, and it is more or less curved, so 
that the sinus on its lower side grows less pronounced in M. marmorata (fig. 30a) 
and fulva (b), more in M. rustica (c) and responsivus (d). Its colour is yellowish or 
brown. 
The radulae (figs. 31—34) have a tricuspid median tooth and bicuspid lateral 
and marginal teeth of different number, most in M. marmorata (about 150: 60.13. 
1.13.60, fig. 31); M. fulva (fig. 32) has 25.13.1.13.25, M. rustica (fig. 33) 
26.14.1.14. 26, M. responsivus (fig. 34) about 33.12.1.12.33. In MW. marmorata 
