302 Transactions. — Zaology, 



The internal coxal plates (i. ex. pi.) have the same relations as in P. 

 edwardsii, indicating imperfect fusion of the coxocerites : external coxal 

 plates are also present. There is, however, no trace of procephalic pro- 

 cesses. 



In P. trigonus, another of the Lcmgoustes ordinaire*, I am informed hy 

 Mr. Haswell that there is a prominent squarish rostrum covering the oph- 

 thalmic segment, hut that in other respects the resemblance to P. vulgaris 

 is very close, there being a well developed stridulating organ, and no 

 clasping processes. 



In P. interruptus, as well as in all other Langoustes longicornes which 

 I have examined or obtained descriptions of, there is a stridulating organ 

 of essentially the same structure as in P. vulgaris. The antennulary sternum 

 (figs. 3 and 6, ant. st.) is, however, truncated in front instead of pointed, so 

 that no part of it overhangs the articular ends of the antennules, which 

 are therefore visible in a view from above. Moreover, the antennulary 

 sternum is nearly as broad in its antero-ventral as in its postero-dorsal 

 region, so that the basicerites (be) are kept widely separated, and do not, 

 as in all the Langoustes ordinaires, partly hide the bases of the antennules 

 when the animal is viewed from above. 



As in P. vulgaris, there is no trace of procephalic processes. The rostrum 

 also is completely aborted. 



The fused coxocerites present no trace of the median groove mentioned 

 in the preceding species, and in correspondence with this the internal coxal 

 plates are absent (fig. 3), so that fusion of the coxocerites is complete. 

 Instead of there being two distinct external coxal plates, one for each coxo- 

 cerite, there is a single transverse sub-vertical posterior coxal plate (p. ex, ]d) 

 consisting of two lamina?, an anterior formed by the posterior walls of the 

 coxocerites, and a posterior furnished by the epistoma (ep). 



Coming now to the general bearing of these facts, it seems clear, 

 to begin with, that the Langoustes ordinaires do not form a natural 

 assemblage, since some of the species included in the sub-genus have a 

 well-developed rostrum, simple unmodified antennulary sternum, etc., 

 while others possess a perfectly formed stridulating organ, — always a 

 mark of high specialization, rare under any form in Crustacea, " ;: and in the 

 present case of a quite unique type of construction. 



The longicornes on the other hand form a strictly natural group, 

 since they all possess the stridulating organ and long antennulary flagella, 

 while none have any trace of a rostrum. 



* See Wood-Mason, Proc. Entom. Soc. Lond., Nov. 1877, p. xxvii. 



