88 On the genus Onchidium. [No. 2, 



No. I. On the genus Onchidium, with descriptions of several new 

 species ; (with plates XIV and XV.) 

 Order. — Pulmonata. 

 Family. — Onchidiid^;. 

 Genus. — Onchidium, Buchannan, 1800. 



Char. Body oblong, entirely covered by a coriaceous, more or less 

 tuberculated mantle, projecting at the sides and internally fleshy ; foot 

 long, narrower and little shorter than the mantle ; head large, distinct ; 

 the mouth situated below, forming a longitudinal slit surrounded by 

 thickened lips, and two, more or less, prolonged and thickened buccal 

 appendages, to the upper edge of which are, so to say, the tentacles 

 soldered on, being represented merely by thickened rims ; superior to 

 these are the long, retractile pedicles bearing on their tips the eyes. 

 Two cartilaginous plates in the oesophagus are covered with a broad 

 radula furnished with very numerous, small equally formed teeth, the 

 central tooth being pointed and equilateral, the laterals usually 

 somewhat smaller, almost all of equal size, slightly hooked, claw-shaped ; 

 no special upper jaw is present. Anus situated at the upper basal end 

 of the foot ; pulmonary orifice posterior to it in the mantle. The sexes 

 are united, the common sexual opening being placed more or less close 

 to the right of the anus, in the fold between the inner side of the 

 mantle and the foot ; a special male organ is besides situated under the 

 right eyepedicle ; it is thick, long, provided with a short flagellum ; 

 the propagation is effected by mutual reciprocal impregnation. Shell 

 none. Habit similar to that of the Limaces, or rather more to that of 

 sea slugs, as I shall endeavour to prove hereafter. 



Before entering upon a description of the various species, it will be 

 necessary to give a detailed statement of the most important and charac- 

 teristic anatomical and morphological points. I select for this purpose 

 the type species of the genus ; any differences in the other species from 

 this type can be afterwards much easier recorded, without giving a 

 repetition of those details. In conclusion I shall allude to the genera 

 Onchidella and Peronia, which have been considered as distinct from 

 Onchidium. 



The upper part of the body of all the Onchidia is, as stated above, 

 always entirely covered by a more or less coriaceous mantle, the epi- 

 dermis of which chiefly consists of a chitinous or horny substance, and 



