212 OSCANIUS. 
larger shells than the larger species. May it not be that the young 
animals have the shell relatively or perhaps absolutely larger than 
the adults, so that the young, especially when they differ also in 
coloration, have been hitherto considered separate species? The 
degeneration of the shell during the life of the individual is well 
known to occur in the Nudibranchs, to a still greater extent. 
(Martens). 
Genus OSCANIUS Leach, 1847. 
Oscanius LEAcH (in Gray, P. Z. 8., 1847, p. 163, L. membrana- 
cea Mont.) Synopsis Moll. Gt. Brit., pp. 28, 29 (1852), type O. argen- 
tatus = tuberculatus Meckel.—VayssIERE, Rech. Moll. Opistobr., 
Tectibranches, p. 121 (Ann. du Mus. d’Hist. Nat. Marseille, Zool., 
ii, 1885).—Susania Gray, Guide Syst. Dist. Moll. B. M., pt. 1, 1857, 
p- 202 (based on O. testudinarius and O. reticulatus). 
Pleurobranchide with the body oval, convex, the mantle of the 
same shape, covering all or a considerable part of the upper surface, 
projecting and overhanging on all sides, more or less distinctly 
notched in front or behind or both; gill bipinnate, the rachis tuber- 
culate. Female generative orifice in front of the gill-insertion, the 
male orifice or penis more anterior, separated from it by some dis- 
tance. Shell as in Pleurobranchus, wholly concealed in the closed 
mantle, sometimes wanting. Type O. tuberculatus Meckel. 
Oscanius includes a few large species, in which the mantle is gen- 
erally tuberculate and sinused in front and at the rear, the gill-stem 
is nodose, and the penis separated from the female orifice by a con- 
siderable space; the latter being the most important character of 
the group. 
The anatomy of the European species has been ably worked out 
by Vayssiére. A number of Indo-Pacific species are herein referred 
to this genus, and the synonymy of the Mediterranean forms is now 
for the first time elucidated. 
Geographic Distribution of Oscanius. 
European: O. tuberculatus, O. testudinarius. 
Indo-Pacific: O. marinus, Red Sea. 
O. mamillatus, Mauritius. 
O. grandis, Huahine. 
O. blainvillii, Tahiti. 
O. violaceus, Sandwich Is. 
Australian: O. hilli, New South Wales. 
