490 Zoological Society : — 



upon the right side being somewhat larger than that on the left ; 

 the surface is smooth and even, and the dactylos is furnished with a 

 prominent carina that terminates abruptly near the basal articulation, 

 and loses itself gradually towards the apex. The fifth pair of pereio- 

 poda are completely hid from view ; the three basal joints are short ; 

 the two terminal ones subequally long, and furnished with a copious 

 brush of strong cilia. These appendages are folded together and 

 enclosed within the branchial chambers, where they, no doubt, fulfil 

 the office of the fiabella of the highest forms of Crustacea — affording 

 an interesting illustration of an organ being converted, by the force 

 of circumstances, from its original purpose to the fulfilment of an- 

 other, for which it was apparently most unsuited. 



Petalocerus bicornis, n. s. 



Carapace triangular, anteriorly produced into two horizontal horn- 

 like processes ; tuberculated with nodulated prominences all over 

 the surface, but furnished with a series of large tubercles correspond- 

 ing in line with the external margin of the carapace ; the antero- 

 lateral margin constricted between the branchial and hepatic regions, 

 furnished posteriorly to the orbit with two strong, blunt processes, 

 and, posteriorly to the central constriction, armed laterally with two 

 distant narrow processes, and posteriorly with six closely situated, 

 large, round tubercles. 



The pleon is nearly symmetrical, being rather larger on the left 

 than the right side. Each segment is defined by a marginal promi- 

 nence ; that upon the left side is continued from near the middle to 

 a process that terminates in a point or tooth at the side, but that on 

 the right becomes confluent with a posterior ridge, and forms an irre- 

 gular circle, the centre of which is deeply depressed. 



The eyes are small, of a green colour, and surmounted on denti- 

 culated peduncles. The first pair of antennae consist of three equal- 

 lengthed joints (of which the first is the more robust), together with 

 a short, stout, pilose flagellum and a slender secondary appendage. 

 The second pair of antennae have a compound scale, consisting of 

 two large and two short compressed processes, and the third joint is 

 furnished with two or three sharp, strong processes. 



The first pair of pereiopoda are chelate and strong, echinated 

 Avith blunt-pointed spines, and terminate in fingers that are flattened 

 at the extremity, and furnished upon the outer surface with numerous 

 tufts of hair, that spring from the summits of the numerous tubercles 

 that are found there. The second, third, and fourth pairs of pereio- 

 poda are more slender than the first, resemble one another very con- 

 siderably, and are furnished with short, sharp, and slightly curved 

 dactyli. The fifth pair of pereiopoda are rudimentary appendages ; 

 they consist of but five joints, the last of which terminates in a blunt 

 extremity that is furnished with a considerable brush of hair, and is 

 probably used for the purpose of cleansing the branchial appendages. 



The pleopoda are present in the female, with the exception of the 

 first pair (which are small) only upon the left side of the pleon, as 

 exemplified in our specimen. 



