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XXXI. Horae Carcinologicae, or Notices of Crustacea. I. A Monograph of the Leuco- 

 siadae, with observations on the relations, structure, habits and distribution of the 

 family ; a revision of the generic characters ; and descriptions of new genera and 

 species. By Thomas Bell, Esq., V.P.B.S., Pres. L.S. 8fc. 



Read June 5th, 1855. 



1 HERE is not perhaps another family amongst the whole of the Decapodous Crustacea 

 so distinctly isolated hy its general characters as the Letjcosiad^e. Belonging, as they 

 obviously do, to the large tribe of Oxystomata, to which they are allied by the important 

 character of the form of the buccal cavity and the structure of the foot-jaws, they have in 

 many respects but little tangible affinity with any other family of that group, by means of 

 intermediate aberrant forms, with the exception of a certain prima facie approach to the 

 Calappadm in the genus Oreophorus, to which further allusion will be made. 



And not only is there such a remarkable absence of any osculant form within the limits 

 of this very natural group, but there is a no less striking want of any obvious approxima- 

 tion to this type in the other families of the Oxystomata ; for the relation suggested by 

 De Haan of the genus Matuta, or rather his family of Matutoidea comprising Matuta 

 and Sepatus, as leading to the Leucosiadce, appears to me quite devoid of any sound 

 foundation. Still less appearance is there of any important approximation to the Bani- 

 nadce, as suggested by the same learned writer. 



I cannot, however, but believe that there is a structural approach to this family in a 

 genus which has hitherto been placed at a remote distance from it by all the authorities 

 on this subject, and particularly by Professor Milne-Edwards in his recent admirable 

 treatise, as it may be called, on the Catametopa. I allude to the genus Pinnotheres, which 

 in the work above mentioned is associated with the families Grapsidce, Gecarcinidm, Ocy- 

 podidce, &c, to which groups its affinity is probably much more slight than to the family 

 now under consideration. It will be reserved for a future occasion to examine into the real 

 relations of the somewhat anomalous family of the Pinnotheridce ; but I would observe, 

 that the general aspect of the male Pinnotheres is so similar to that of a true Leucosia 

 as to be obvious at the first glance ; and although I would not trust too much to external 

 form and general character or aspect, I think these points may be, and often are, under- 

 rated. There is frequently a physiognomical character, so to speak, which is indicative of 

 some close relation of affinity, which ought not to be cast aside hastily and without due 

 consideration as a mere analogical resemblance. I shall not on the present occasion enter 

 into a detailed investigation of the relation between these two types, but I may observe 

 that the form of the buccal opening, the foot-jaws, the eyes, the antennae and other im- 

 portant organs appear to me to afford indisputable indications of the affinity in question. 



The approach of the genus Oreophorus to the Calappadce is, however, more apparent, 



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