220 THE VOYAGE OF H.M.S. CHALLENGER. 



Tribe Penaeidea. 



The numerous genera which belong to the several families that form this tribe 

 possess in common certain peculiarities of form and structure that distinguish them 

 essentially from all others. 



The earlier writers, including Professor Milne-Edwards, grouped within it all species 

 in which the several legs posterior to the oral appendages carry more or less per- 

 fectly developed branches attached to the second or basisal joint. This he thought 

 to be so important and distinguishing a feature, that he considered it sufficient to 

 determine the tribe, and recently Professor Sars, in his Report on the Schizopoda of 

 the Challenger collection, considers it as " perhaps the most striking feature distinguish- 

 ing" the Schizopoda as a distinct suborder. Thus the genus Optlopihoriis, which in 

 most other respects approximates to the Palaemonida3, was by Milne-Edwards associated 

 with the Pemeids, as well as other less known forms, such as Euphema, JEphyra, 

 Pasiphzea, &c, but which de Haan, and following him, Dana, have excluded. Still more 

 recently, Professor Huxley, in his article on the classification and distribution of the 

 Crayfishes, 1 has suggested the elimination of Stenopus also, on account of the dissimilarity 

 of the branchial structure ; and upon the same evidence the genus Spongicola must 

 also be excluded. 



Family PeNjEidj;. 



The structure of the Penseidas offers so many points of interest, that it appears to 

 afford an instructive lesson to compare their several parts with those of the Astacidea 

 and Caridea, so as to recognise the points in which they agree, as well as those in which 

 they differ. 



Taking the genera of this family as being the most perfectly developed of the 

 tribe, from which others are but departures to a greater or less degree, we generally 

 find that the animals are laterally compressed, and that this compression increases 

 posteriorly to the last somite of the pleon. 



The carapace is well developed, and laterally deeply produced ; posteriorly it is 

 carried further back at the sides than in the median dorsal line, and passes under the 

 anterior margin of the coxal plates of the first somite of the pleon. 



The dorsal median line is carinated, but in some forms the carina terminates with 

 the posterior extremity of the rostral crest. The rostrum is always laterally compressed, 

 and is generally long and frequently strengthened on the sides by a longitudinal ridge. 

 In some genera, such as Gennadas and Bcnthesicymus, the rostrum is short, and 



1 Proc. Zool. Soc. Loud., vol. iv. p. 780, 1878. 



