278 MR. BELL, HOR^ CARCINOLOGIC^ ; 



and in all probability constitutes a true relation of afiinity. This remarkable genus was 

 first formed by Dr. Riippell, who figures and describes a species, 0. horridus*, found by 

 him in the Red Sea. It is evidently constructed on a type approximate to the CalapimdcB. 

 Like them it has the power of concealing the feet under the body, so that when at rest 

 they are protected by the margin of the carapace, which is somewhat dilated laterally. 

 In this particular it resembles jEthra, as well as Cryptopodia, Lambrus and others, with 

 which it has however no other near structural relation. The genus Niirsia of Leach, 

 and the new genus Lithadia, which is closely aUied to Ebalia, approach it in a slight 

 degree in this respect, and the latter still more in its general aspect and the extreme 

 rugosity of the body. 



The tendency to a lateral dilatation of the carapace is indeed a very striking character 

 in several forms of tliis family. It has been already alluded to in reference to Oreophonis 

 and lAthadla : it appears also to a certain extent in Phlyxia and Ebalia. But in Iphis 

 it assimies a very different form, terminating in a long acute spine on each side, recalling 

 in some measure the aspect of the genus Matuta ; whilst in Ixa a still more remarkable 

 development is observed in an extraordinary lateral extension of the carapace itseK, which 

 is twice as broad as it is long, besides its still further production into a somewhat cylin- 

 drical process on each side, the two processes together constituting about half the total 

 breadth. 



The characters of a group so distinctly marked could not fail to strike the accurate and 

 observant mind of Fabricius, who, in the course of his re-formation of the whole class, 

 brought together all the species which were then known of the Leucosiadce into a single 

 genus, to which he gave the name of Leucosia. This name was retained by Leach for the 

 form which is evidently the typical one ; and he arranged into several well-defined generic 

 groups the species thus associated by his predecessor, together with others with which he 

 had become acquainted. All Leach's generic divisions have received the sanction of 

 subsequent naturalists ; and the only changes which have been introduced since his time 

 have consisted in the discovery of some new speciesf, if we except the mistaken application, 

 by Milne-Edwards, of the name of Giiaia to certain species which Leach had akeady 

 designated under the generic appellation of Fersephona. This mistake however was a 

 very natural one, arising from the vague and brief terms in which Leach had indi- 

 cated rather than described or defined them. The specific and even the generic characters 

 given by this distinguished naturalist are often, from an inordinate desire for brevity, 

 extremely vague and incomplete ; and now that the number of known species and of 

 generic forms has become so immensely increased, it is often in vain that we endeavour 

 to reduce to any certainty the contracted and indefinite phraseology in which his characters 

 are expressed. 



It is clearly of the greatest importance not only that the distinctive phrase applied to 



* Krabben der Rothen Meeres, p. 18. t. 4. f. 5. 



+ Harrovia and Tlos, genera described by Adams and White in the " Crustacea^' of the Voyage of the Samarang, 

 and placed among the Leucosiadce in that work, certainly do not belong to this family. Iphieulus is stated by those 

 authors to belong to the Parthenopida, although located in their work with the LeucosiadeB i but as regards this genus, 

 I am led, by examination of the specimens in the British Museum, to the conviction that it is in truth a Leucosian genus, 

 and that they are right in the text and not in the note. 



