REPORT ON THE CRUSTACEA MACRURA. 227 



In Sergestes and its congeners it is elongated and pediform, and if not the longest is 

 cert a i nl v the most efficient appendage attached to the pereion. 



The second pair of gnathopoda is long, pediform, and invariably supports a multi- 

 articulate branch that springs from the basis or second joint, while the first or coxa 

 carries a long mastigobranchial lash, that in Penseus is more or less divided in a fork- 

 like manner ; in some genera it is long and leaf-like, in others it is oval, but 

 generally the body of the lash is supported by a stalk to which it is attached at an angle. 



In many genera this lash supports a well-formed branchial plume, but it is absent 

 in Penseus, Sicyonia, and Artemesta, whereas in Haliporus it is present in a reduced 

 condition. 



The first three pairs of pereiopoda are formed on the same typical plan ; they 



gradually increase in length as they recede posteriorly, and also slightly decrease in 

 diameter. One feature that distinguishes them from those of nearly all other forms is 

 the great length of the carpos in relation to the propodos, and outside of this tribe, I only 

 know of three genera, the recently found Nematocarcinus, belonging to the Tricho- 

 branchiata, and Stenopus and Spongicola — genera that belong to the Phyllobranchiata, 

 and have long been supposed to be associated with the Penseidse — that possess this 

 feature, which they do in each pair of pereiopoda except the third, and here the great 

 increase of development of the chela is the apparent cause of its absence. All the 

 true Penceidas carry a mastigobranchial plate, while the Sergestidse have none except in 

 those genera where a rudimentary plate is attached to the first pair of gnathopoda. 

 In Penseus and its near congeners there is no podobranchial plume, while in Benthe- 

 sicymus there is one to each pair of appendages except the posterior pair. The two 

 posterior pahs in this genus are simple, and terminate in a styliform dactylos. 

 Penseus carries neither lash nor podobranchial plume, but a mastigobranchial lash 

 appears in several other genera, but never a podobranchial plume. 



In the Sergestidse there is neither lash nor plumes attached to the legs, but the 

 rudiments of both are sometimes attached to the first pair of gnathopoda. 



The branchial arrangement in the several genera of the Penseidae may best be 

 appreciated by being shown in a tabulated form : — 



Penseus. 



Pleurobranchiae, . . .1111111 



Arthrobranchise, . . .222221... 



Podobranchiae, . . 



Mastigobranchise, . . . 1 1 1 1 1 



h i k 1 m n o 



Penseopsis. 

 No species in tbe collection. 



