188 C. Spence Bate on the Penceidea. 
of gnathopoda and the three anterior pairs of pereiopoda), 
whereas in Pencus there are none; and if comparison be 
made with Duvernoy’s figure of the branchial arrangement, 
it will be seen that he represents no mastibranchial lash 
attached to the penultimate pair of pereiopoda, whereas the 
specimens that have come under my examination in the 
‘Challenger’ collection show that there is a large mastibran- 
chial plate. In discussing this pomt with M. A. Milne- 
Edwards, he contended that the presence or absence of an 
appendage such as this, unless it were corroborated by some 
external evidence, was a very doubtful specific character, and 
one that was of no value in the history of descent when not 
otherwise illustrated. 
Still it appears to me that a feature relating to the economy 
of the animal of so important a character, if constant, must be 
of specific value; so that its generic connexion must depend 
upon its constant character in allied specific forms. If on 
further examination I am enabled to determine a series of 
species in which the mastibranchial lash is constant, however 
much in other respects they may resemble Aristeus, they must, 
in our present state of knowledge, be arranged as a distinct 
genus; and as I think this can be done, I propose provision- 
ally the name of Plestopeneus for such as have the masti- 
branchial plate attached to the penultimate pair of pereiopoda, 
which is not present in the figure given by Duvernoy (“Crus= 
—taeés-de_Nice;” Ann. des Sc. Nat. vol. xv. 1841). 
Aristeus antennatus, Risso. 
Rostrum styliform, reaching a little beyond the extremity 
of the peduncle of the first pair of antenne, and with three 
strong teeth on the dorsal surface at the base. Pleon smooth, 
fourth and fifth somites produced to a poimt on the dorsal 
surface in the median line, but not elevated or produced into a 
tooth. 
Hab. Mediterranean, at a great depth (isso, Duvernoy, 
Johnson), Algiers. 
Peneus Edwardsianus of Johnson (Proc. Zool. Soc. 1867, 
p- 897) undoubtedly belongs to the same species. 
Aristeus armatus, 0. Sp. Picco rr 
Rostrum styliform, as long again as the peduncle of the 
first pair of antenne. Three long teeth near together above 
the frontal margin. Third, fourth, and fifth somites of the 
pleon dorsally produced into a laterally compressed tooth ; 
sixth somite carinate. ‘Telson armed on each lateral margin 
with four small spines. Ophthalmopod long. Eye not large. 
