52 On a new West-Indian Tanaid. 



From an unmounted specimen with which Mr. Forrest has 

 favoured me since the above description was passed for press 

 it appears that the lateral margins of the head anteriorly are 

 slightly concave, that the first three free segments of the 

 perason are very decidedly shorter and a little broader than 

 the following three, that, viewed dorsally, there is a constric- 

 tion between the third and fourth and between the fourth and 

 fifth free segments, and that the first five segments of the 

 pleon are slightly broader than the immediately preceding 

 segments of the perseon. In both specimens the mouth- 

 organs appear to be in a very rudimentary condition. 



In his recent contribution to the Crustacea of the Plankton 

 Expedition Dr. H. J. Hansen gives some weighty reasons 

 for adopting the view that the families of the Apseudidse and 

 Tanaidae should form a separate order, which he calls Tanai- 

 dacea, coordinate with the Amphipoda, Isopoda, and Cumacea. 

 I shall not easily be convinced that the new order is required. 

 It is much more a question of convenience than of scientific 

 accuracy. Though the two families in question have points 

 in common with the Amphipoda and Cumacea, they are 

 trenchantly distinguished from both of those groups. On the 

 other hand, they have the dorso-ventrally depressed body 

 which is so prevalent among the Isopoda, and in detail they 

 show several points of agreement with various members of 

 the Isopodan families. Thus the peculiarity in Apseudes that 

 the second antennas have an exopod has its parallel in Stene- 

 trium and Janira. In Stenetrium, Haswell, and Pkreatoicus, 

 Chilton, the first gnathopods have a prehensile hand to some 

 extent equivalent to that found in the Apseudidae and Tanaidas. 

 In the Gnathiidge and the Cryptoniscian forms among the 

 Epicaridea there occur pleopods strongly resembling forms 

 of those appendages in the families just mentioned. The 

 uropods of the Tanaidaa cannot be considered remote from those 

 of the Asellida?; and the coalescence of the first perEeon- 

 segment with the cephalon is exhibited in a more or less 

 marked degree in the male sex of the Gnathiidas. 



In regard, then, to the acceptance of the proposed new order, 

 Tanaidacea, in exchange for the group of the Isopoda Cheli- 

 fera, the important question arises, cm bono ? For whose 

 advantage will the change be made ? So long as the higher 

 classificatory divisions are capacious, they hold out a welcome 

 to new discoveries ; but when their boundaries are contracted, 

 the next new species is liable to find itself left out in the 

 cold, and then perhaps a new order must be established for 

 solitary form. Moreover, when the breaking-up of a fairly 



