108 On the genus Onchidium. [No. 2, 



which enclose the central dorsal area. The entire mantle is more or 

 less finely granular. All the tubercles are much less developed in young 

 specimens, and even in old ones their form constantly changes, on 

 account of the softness of the body, in which they can be entirely 

 retracted, making the mantle to appear uniformly convex. Young 

 and half grown specimens generally have on the external side of the 

 dorsal ridges, two or three of the blackish spots larger, separated by 

 oval pale orange spots which sometimes are partially confluent, form- 

 ing longitudinal stripes, the orange colour also partially extending 

 on the ridges themselves. 



The mantle below is uniform pale greenish grey, with very minute 

 and numerous white dots, the same being also traceable on the sides 

 of the foot. The latter is blackish green, little shorter than the mantle, 

 obtuse or slightly rounded in front, pointed at the posterior termi- 

 nation when free, but when the animal moves about on a flat surface, 

 it appears rounded. The width of the foot is on an average f- th of 

 that of the body, occasionally somewhat less. The head is very large, 

 greenish, covered in front with numerous ashy warts : the buccal 

 appendages laterally widely expanded, with the front edges slightly 

 swollen, the tentacular rims above them being very thin, and of an 

 ashy grey colour. The eye -pedicles are stout at the base, when 

 extended about half an inch long, slightly warty, concentrically wrin- 

 kled, with the tips distinctly swollen, globular, pale yellowish or 

 reddish, bearing the black eyes almost centrally situated in a lighter 

 transverse fold. The lips of the mouth are whitish, strongly thicken- 

 ed and folded. The anus is as usually placed at the upper terminal 

 base of the foot ; the pulmonary orifice is removed from it and close 

 to the posterior end of the mantle ; it is large, surrounded by a strong 

 swollen margin, internally white, with 8 — 10 small tubercles, which 

 continue interiorly as short ridges. The hermaphrodite pore is also 

 somewhat removed from the anus, about half an inch distant from it 

 to the right, but situated as in all other Oncliidia in the fold between 

 the mantle and the foot. The external vas deferens is a distinct narrow 

 groove, entering the body at the outer base of the right oral append- 

 age, although it seems to continue below the mouth, issuing internally 

 quite close to the penis opening. The penis pore itself is large, 

 placed laterally below the right eye-pedicle. 



