298 Transactions.— Zoology. 



An examination of several species of Palinurus has shown that while 

 the stridulating organ is present in all the Langoustes longicornes associated 

 with long antennulary flagella and ohsolete rostrum, there are some Lan- 

 goustes ordinaires in which it is well developed, others in which it is wholly 

 absent ; and further that in the latter group some species have a rostrum 

 quite comparable with that of the Astacida, others having one so small as 

 to merit special description only from its position. 



I propose to describe, somewhat in detail, the structure of the head in 

 P. edwardsii, P. vulgaris and P. interruptus, in order to bring out the points 

 of likeness and of difference in three typical forms. 



In Palinurus edwardsii, one of the common New Zealand species, the 

 carapace is produced in front into an upturned rostrum (figs. 1 and 4, r) 

 which is confluent laterally with the large supra-orbital spines (s. or. sp). 

 The ventral faces of these latter pass insensibly into the " epimeral plates " 

 (epm. pi.)* which form the posterior walls of the orbits, and are fused exter- 

 nally with the anterior border of the carapace and internally with the 

 antennulary sternum (ant. st.). This latter is a vertical bar, just wide 

 enough below to furnish the articular facets for the antennules (ant. 1'), and 

 narrowing to its dorsal end where it becomes bent backwards at a right 

 angle (fig. 1) to join with the epimeral plates. The latter, which are con- 

 sidered by Huxley as representing antennulary epimera,f bound a trans- 

 versely oval area, consisting of uncalcified chitin, and bearing the small 

 movable ophthalmic segment (oph.) or pseudo-segment, the middle portion 

 of which is calcified, forming the so-called ophthalmic sternum. The epi- 

 meral plates are thus united with one another above and below the eye- 

 bearing space forming supra-ophthalmic and infra-ophthalmic bars. 



The infra-ophthalmic bar gives off on each side of the middle line a 

 pedate process (cl. pr) which extends forwards in contact with, but quite 

 free from, both its fellow of the opposite side and the dorsal or horizontal 

 portion of the antennulary sternum. Each of these processes ends in a 

 sharp spine and, from its proximal end, sends off an upwardly directed off- 

 shoot ending in two small spines, which is so closely applied to the lateral 

 surface of the rostrum that the latter appears to be actually squeezed 

 between the two clasping processes as they may be conveniently termed 

 (cl. pr, figs. 1 and 4). 



These clasping processes appear to have been very imperfectly described 

 hitherto. Milne-Edwards merely says that there are two small spines 

 below the base of the rostrum (in P. lalandii which agrees in all important 



* Huxley, " The Crayfish," p. 156, fig. 40. 



t Is it not more likely that these plates represent the ventral region of the unseg- 

 mentecl praestomium ? If embryology should answer this question in the affirmative it 

 will be convenient to speak of a single prcestomial plate consisting of lateral, supra - 

 ophthalmic, and infra- ophthalmic portions. 



