26 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



half the length of the penis ; spermatheca oval, with a long duct. 

 The free oviduct is shorter than the vagina ; the uterus and albumen 

 gland are without peculiar features. The ovisperru duct penetrates 

 the cavity of the mantle. The ovotestis consists of numerous short, 

 stout caeca, situated at the posterior edge of the liver within the dorsal 

 cavity (see PL III, Fig. 4, o.t.). 



Free muscles. — The retractor of the penis is inserted upon the 

 diaphragm (PI. Ill, Fig. 8, p.r.). The columellar muscle is very 

 short (PI. Ill, Figs. 2, 4, col.), and inserted upon the columella of the 

 shell close to the apex, or upon the mantle lining this part of the 

 ventral surface of the shell. Pefore emerging from the mantle- 

 cavity it splits into two equal branches (PI. Ill, Fig. 4, viewed from 

 below). The right branch is simple for half its length, then dividing 

 into ocular, tentacular, and labial (?) branches. The left retractor 

 splits far nearer the insertion, the buccal retractor branching off ; 

 and much further forward it again divides into ocular and tentacular 

 retractors. There are no caudal or pedal retractors. 



Lung rather small, with a spongy reticulation near the respiratory 

 orifice, elsewhere appearing smooth. 



Kidney placed against the liver, on the right side and partially 

 above it. (See PI. Ill, Fig. 2, k, kidney seen from below, the liver 

 removed; and Fig. 3, kidney from above, the attached face shaded.) 

 The ureter was not seen, but probably follows G 4 . 



Heart (PI. Ill, Figs. 2, 3) surrounded on the sides and below by 

 the kidney, but the pericardium is visible above (Fig. 3). 



Distribution. — Mexico (State of Michoacan). 



Compared with Xanthonyx, Metostracon shows numerous similarities, 

 but it is decidedly more slug like. The shell has almost entirely 

 lost the spiral form, and is completely buried in the mantle. The 

 genitalia, while of the same general type, differ in details, the mucous 

 glands in Metostracon being sac like and inserted directly upon the 

 dart-sac, as in Cepolis, while in Xanthonyx the glands are oblong, 

 with slender ducts, which are inserted at the junction of the dart-sac 

 with the vagina, as in Micrarionta, a subgenus of Epiphragmophora. 

 The relative positions of the liver, kidney, and heart are also very 

 different in the two genera : in Metostracon the heart being between 

 two lobes of the kidney, and above the liver, while in Xanthonijx the 

 heart lies to the left of the kidney, and the liver does not extend 

 beneath it. 



These differences render it certain that Xanthonyx is in no sense 

 an earlier stage in the phylogcnetic history of Metostracon. The two 

 genera are shown by their anatomy to have descended from belogonous 

 helices, but in rearranging their organs in the more limited space of 

 the diminishing mantle -cavity, each has solved the problem in its own 

 way. It is therefore likely that the divergence between the two genera 

 began at latest almost as soon as the reduction of the mantle -cavity 

 and shell commenced. They may have had a common hclicoid ancestor, 

 or, if not, it is obvious that their respective helicoid progenitors were 

 very closely allied. 



