542 MR. K. J. MiKRS ON [Juiie C, 



prominences on which the genital apertures are placed are quite short 

 and unarmed. 



Reference should have been made in my previous paper on 

 Mauritian Crustacea obtained by the British Museum from M. 

 Robillard, to tw o species which were received from him last year. One 

 of these is a fine and large specimen of an Alpheus, which I refer some- 

 what doubtfully to the Alpheus rapax of Fabricius, as described by De 

 Haau, It has the rostrum prolonged backward as a median dorsal carina 

 to the middle of the carapace, andin other particulars agreesexcellently 

 well with De Haan's description', but differs from the figure in 

 having the mobile finger of the larger chelipede vertically deeper 

 than in that author's figure and subtruncated at its distal extremity. 

 The other species is an apparently undescribed species o( Pseudibacus, 

 Guerin-Meneville, a genus previously unrepresented in the collection 

 of the British Museum. 



PsEUDiBACUs PFEFFERi, sp. n. (Plate XXXVI. figs. 2, 3.) 

 The carapace is broader than long ; the ambulatory legs are con- 

 cealed beneath its lateral prolongations. The upper surface of the 

 carapace is nearly smooth ; the antero-lateral margins are thin, acute, 

 and regularly crenulated on each side; a notch marks apparently the 

 line of demarcation between the cervical and postcervical regions ; in 

 front of the lateral notches are seven or eight,and behind them eleven or 

 twelve crennlations ; alongitudinal depression exists on each branchial 

 region, not far from the lateral margins. The median dorsal line of 

 the carapace is marked by a longitudinal ridge or keel, which origi- 

 nates at a short distance behind the anterior margin, and on the 

 cardiac region is bifurcated — the two carinse thus foriiied enclosing 

 a median depression or shallow pit, behind which they reunite and 

 are continued as a simple ridge to the posterior margin of the cara- 

 pace ; the anterior and the two median carinse are each divided into 

 three or four obscurely indicated lobes ; the posterior carina is entire, 

 but terminates in a small tubercle on the posterior margin of the 

 carapace. The anterior margin of the front is bisinuated in its 

 median portion, where the basal lobe of the rostrum is articulated 

 with it ; the terminal plate of the rostrum is transverse, with the 

 extremity subacute, and is divided through about half its length by 

 a median, longitudinal, neai'ly closed fissure. The orbits are situated 

 about midway between the median line and the lateral angles of the 

 carapace, and are merely closed by a process of the basal antennal 

 joint &s in Pseudibacus fferstaecAeri. The postabdomeu resembles 

 that of P. gerstaeckeri as described by Dr. Pfeffer in all essential 

 characters : thus the second to fifth segments are each armed with 

 a median dorsal carina, which slopes obliquely downward to the 

 anterior margin (except on the second segment), and is posteriorly 

 produced and ends in an acute tooth on the fourth and fifth segments; 

 the sixth segment has a small median tubercle on its posterior mar- 

 gin, and the seventh three at the line of demarcation between the 



' Vide Crustacea, in Siebold's 'Fauiia Japonica,' p. 177, pi. liv. fig. 2 (1849). 



