REPORT ON THE CRUSTACEA MACRURA. 107 



excluded. This valve is a thick and strong appendage, and seems to vary but little. The 

 variation that does take place appears to be in the form of the mastigobranchise, or the 

 posterior extremity of the great squamose plate ; and in the smaller of the two branches, 

 which in some species, as in Stereomastis suhmi, is short and pointed. The large branch 

 is generally folded longitudinally on itself. This appendage, except in possessing a strong 

 mastigobranchia, a part common to most forms of Crustacea, varies considerably from 

 the corresponding member in the Scyllarida?. and also in the Astacidaa, but approximates 

 to that in Palinuridre. 



The third pair of siagnopoda (maxillipedes) (PI. XVIII. g) appears to be an im- 

 portant pair, if we may judge by its relative proportion. It is situated behind the mouth, 

 on one side of the median line, and reaches back into the branchial chamber as far as to 

 the extremity of the anterior branchial plume, and anteriorly beyond the oral apparatus, 

 so that its extremity is visible in advance of the frontal margin of the 

 cephalon. It consists of several branches, flat and leaf-like in character, 

 which are so differentiated from the typical form in Crustacea, that it is 

 onlv by an analytical comparison that the several parts can be homolo- 

 gically determined. The chief or primary branch is of great tenuity, 

 and is folded to form a spoon-like hollow, with its convex surface turned 

 inwards. This siagnopod is implanted immediately behind the mandi- 

 bles, having the anterior pair outside rather than anterior to it. The 

 cup-like hollow, which is formed by the leaf being longitudinally folded 

 on itself, is turned outwards, and its inner or deeper angle, being that F %J^^^^J e X 3 f 

 which is nearest the body of the animal, is subapical to the anterior ^^f^T by 

 extremity ; thus a freely articulating plate plays in the cavity with 

 probably a more or less constant voluntary vibration. This vibrating plate keeps the 

 water circulating within the branchial chamber ; while the mastigobranchial plate, which 

 is very long and broad, and generally free from hairs, overlies that of the second pair, and 

 assists it not only in its function, but, by permitting a free space between them, allows 

 the water that may have been confined within the branchial chamber to pass out with 

 more or less rapidity. 



These are the several appendages that belong to the cephalon or head. They are 

 much compressed together, inasmuch as there is none in a direct line between the 

 metastoma and the third pair of siagnopoda. The first and second being situated later- 

 ally and but slightly posterior to the mandible. 



The first pair of gnathopoda (PL XVIII. h) shows an approach to the pediform 

 character. It is flat, broad, and covered with hairs, and varies very little in the several 

 species. In Pentacheles euthrix the basis is serrate on the inner margin, while in most 

 forms it is smooth on that part. In Pentacheles euthrix also there is attached to the 

 outer and upper angle of the coxa a small projecting process that I take to represent the 



