278 MR. BELL, HORÆ CARCINOLOGICÆ ; 
and in all probability constitutes a true relation of affinity. This remarkable genus was 
first formed by Dr. Rüppell, who figures and describes a species, O. horridus*, found by 
him in the Red Sea. Itis evidently constructed on a type approximate to the Calappade. 
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 Zthra, as well as Oryptopodia, Lambrus and others, with 
which it has however no other near structural relation. The genus Nursia of Leach, 
and the new genus Lithadia, which is closely allied to Æbalia, 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 this family. It has been already alluded to in reference to Oreophorus 
and Zithadia: it appears also to a certain extent in PA/yria and Ebalia. But in Iphis 
it assumes 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 Iva a still more remarkable | 
development is observed in an extraordinary lateral extension of the carapace itself, 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 Leucosiade 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 species}, if we except the mistaken application, 
by Milne-Edwards, of the name of Guaia to certain species which Leach had already 
designated under the generic appellation of Persephona. 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. 
. Tt is clearly of the greatest importance not only that the distinctive phrase applied to 
* Krabben der Rothen Meeres, p. 18. t. 4. f. é. 
e Harrovia and Tlos, genera described by Adams and White in the “ Crustacea” of the Voyage of the Samarang, 
ela among the Leucosiade in that work, certainly do not belong to this family. Iphieulus is stated by those 
authors to belong to the Parthenopide, although located in their work with the Leucosiade ; but as regards this genus, 
= led, by ne 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, 
