600 CRUSTACEA. 



Subtribe IV. PENiEIDEA. 



While many acknowledged species of the subtribe Penseidea (or 

 " Tribu des Peneens") have no palpus attached to the legs, there are 

 other species, that have none of the true characteristics of the group, 

 excepting the uncertain one, of having this palpus. De Haan first per- 

 ceived the true relations of these supposed Penaean genera, and trans- 

 ferred them to the other divisions, where their affinities place them. 

 The several reasons for the limits we have adopted for this group have 

 been mentioned on a preceding page. It marks a degradation in rank 

 among the Macroura, which degradation is exhibited in two ways. In 

 the higher species of the group, the functions of the first and second pairs 

 of legs are divided with the third pair, this last pair being didactyle, like 

 the second, and of much greater size. In the lower species, the legs all 

 become slender, and none are stout didactyle ; and often the second pair 

 of maxillipeds, or even the first pair, is elongated and pediform, while 

 also the posterior legs become rudimentary, as a result, evidently, of a 

 greater prostration of the forces of life. It is this diffusion of the 

 forces which in the superior Macroura are subcephalic, along the 

 range of the cephalo thorax, that characterizes the species of the Penseus 

 division. Another character distinguishing the Penseoids, and appa- 

 rently another mark of degradation, is the fact that the third abdo- 

 minal segment, instead of having the peculiar condition of overlapping 

 laterally the segment either side, for the greater compactness of the 

 whole, is but one of the common series, being laterally overlapped like 

 the following, by the segment preceding it. 



The mandible in this group is peculiar in having a simple dentate 

 summit, and generally the organ is placed very obliquely, instead of 

 having the summit at all flexed inward. All the species, as far as 

 examined, have a palpus to the mandible, which is either short and 

 stout, or slender in form. 



The Penaeidea include three families, distinguished by the character 

 of the legs and the second and third pairs of maxillipeds. They are 

 as follows : — 



Fam. I. Pen^eidjl — Pedes 6 antici chelati, 3tiis longiores et plus 

 minusve validiores. 



