542 MR. &. J. MIERS ON [June 6, 
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 two species which were received from him last year. One 
of these is a fine and large specimien of an Alpheus, which I refer some- 
what doubtfully to the Alpheus rapa of Fabricius, as described by De 
Haan. It has the rostrum prolonged backward as a median dorsal carina 
tothe middle of the carapace, and in other particulars agrees excellently 
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 of Pseudibacus, 
Guérin-Méneville, a genus previously unrepresented in the collection 
of the British Museum. 
PsEUDIBACUS PFEFFER], 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 sevenor eight,and behind them eleven or 
twelve crenulations ; 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 carinee thus formed 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 carine 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, nearly 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 as in Pseudibacus gerstaeckeri. The postabdomen 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 ‘Fauna Japonica,’ p. 177, pl. lxv. fig. 2 (1849), 
