REPORT ON THE CRUSTACEA MACRURA. 573 



The somites of the pleon are deep and not strongly compressed ; the three posterior 

 somites rapidly narrow to the extremity of the telson, which is ovate, the lateral plates 

 having a short diajresis, the outer angle of which is furnished with two strong spines. 



The branchial apparatus corresponds with that of Paralpheus very nearly, and 

 differs from that of Alpheus only in the absence of the rudiments of the mastigobranchia? 

 attached to the second parr of gnathopoda and all the pereiopoda in that genus, as shown 

 in the following table : — 



Pleurobrancliias, . . . 1 1 1 1 1 



Arthrobranchise, . . . 



Podobranchia?, . . . ... 1 



Mastigobrancbiae, . . . 1 



h i k 1 m n o 



The aspect of the animal as a whole at once gives the idea of its distinctness from 

 Alpheus and also from Paralpheus. The carina in the median line on the dorsal 

 surface, the length of the smooth rostrum, and the production of the fronto-lateral angle 

 of the carapace to a point, are suggestive of a separation from Alpheus, and the form of 

 the mandibles and the absence of the mastigobranchige support this conclusion. But 

 the separation from Paralpheus, although equally important is less conspicuous. 

 The rostrum in both is remarkable for its length, but in Paralpheus it has a 

 feeble tendency to denticulation on the upper surface. The stylocerite in Paralpheus 

 is small, in Synalpheus it is large. There is only one tooth, and that remarkable 

 for its length, attached to the base of the second pair of antennse in Paralpheus, 

 whereas there are two in Synalpheus. 



The mandible in Paralpheus has a strong cylindrical molar process, a broad, flat, 

 serrate psalistoma, and a short, spatuliform, uniarticulate synaphipod, while in 

 Synalpheus the molar process is stout and cylindrical, the psalistoma is almost 

 rudimentary, being little more than a large curved and pointed tooth, and the 

 synaphipod is short and biarticulate. The first pair of gnathopoda carries a circular 

 mastigobranchial plate in Synalpheus and a semicircular one in Paralpheus. The 

 second pair of gnathopoda in Paralpheus is more robust, terminates in a point, and 

 carries a slender uniarticulate basecphysis, while in Synalpheus this pair of appendages 

 is less robust and has the distal two joints shorter than the preceding, terminates in 

 a bifid apex, and carries a long, stout, uniarticulate basecphysis. 



The first pah' of pereiopoda is more asymmetrical in Paralpheus than in Synalpheus. 

 The second pair corresponds in all respects, except in the numerical value of the 

 articuli of the carpos. The third and fourth pairs are larger than the fifth in 

 Paralpheus, whereas in Synalpheus they are correspondingly equal in form and power. 



The rhipidura bears some general resemblance in the two genera, but differs in 

 detail. In ParalpAeus there is one large tooth at the outer angle of the basal joint of 



