100 On the genus Onchidiurn. [No. 2, 



anal and genital openings. Figure 2 on the same plate represents 

 the genital organs, but does not seem to be very correct ; in any 

 case it is not sufficiently clear. 



The reason, — that forms which appeared to such exact observers, as 

 Cuvier, Lesson, Quoy and Gaimard, in all external characters to be 

 generically identical with Buchannan's Onchidiurn, but which were by 

 others separated as distinct genera, — evidently lies in the insufficient, 

 and partially incorrect account which the last named author gave of 

 his newly proposed genus, though very probably the desire of man, to 

 discover new forms, had also something to do with it. Tbe consequence, 

 in short, was that the name Onchidiurn was reserved for the type species 

 Onchidiurn typhce, and other forms which were better known, than this, 

 were separated into distinct genera. Now, when all the anatomical 

 details of the type species are before us, we shall be able to draw a 

 more accurate comparison between the same and other species. 



Cuvier, as I have already stated, gave an excellent account of the 

 anatomy of a Mauritian species which he called O. Peronii. Blainville 

 in the 32nd vol. of the Diet, de scienc. nat, p. 280, proposed for this 

 species the name Peronia Maitritiana* as the type of a new genus. 

 When we compai-e externally the position of the anus, the hermaphro- 

 dite and male genital pores, and the pulmonary orifice, then the form of 

 the head and the eye-pedicles &c, of Cuvier's original drawings, with 

 those given of Onchidiurn typhce, it will be readily seen that no 

 essential distinction between them can be recorded. Even the granu- 

 lation of the mantle is not much stronger, but it is said that the 

 tubercles form (probably during life) short tufts. Referring to the 

 other anatomical drawings, it must be admitted that they shew a 

 perfect identity with those of Onchidiurn typhce, if we set aside some 

 minute details which are not perfectly clear in Cuvier's figure, and which 

 are easily explained, when we consider that "Cuvier had only specimens 

 preserved in spirit for examination, and that many of those minute 

 organs may consequently not have been preserved. Keferstein's and 

 Savigny's figures of O. Peronii or verruculalum also fully agree with 

 the typical Onchidiurn, as far as internal characters are concerned • the 

 only difference being again the presence of tufts in place of simple 



* The rule, that specific names, unless pre-occupied, must not be changed, 

 ought always to be observed. 



