Pabkek. — On the Structure of the Head in Palinunis. 299 



respects with P. edwardsii), and the same statement is made by Miers* and 

 by Haswell.t The processes are perfectly distinct from the rostrum, either 

 being removable without injury to the other, and clearly belong to the 

 epimeral plates, so that if the latter are, as Huxley supposes, antennulary 

 epimera, the clasping processes are to be looked upon as epimeral — or pos- 

 sibly as partly sternal and partly epimeral — outgrowths of the antennulary 

 segment. 



The true relations of the rostrum are very imperfectly seen in an external 

 view : owing partly to the presence of the clasping processes which form an 

 apparent proximal boundary to it, partly to the fact that on its dorsal sur- 

 face it widens out suddenly and becomes confluent with the supra-orbital 

 spines, it appears externally as a very small structure, hardly larger indeed 

 than the clasping processes (fig. 4). But a longitudinal section (fig. 1) 

 shows that it is really a structure of considerable size, being continued 

 backwards some distance behind the clasping processes, and a short distance 

 behind the ophthalmic segment where its ventral plate turns sharply forward 

 and becomes continuous with the supra-ophthalmic bar of the epimeral 

 plates. 



A short distance on each side of the middle line the supra-ophthalmic 

 bar is produced into a small hollow procephalic process (fig. 1, pre. p.) which 

 passes backwards, upwards, and outwards into the interior of the head 

 quite like the homologous structure in Astacus and Homarus, from which it 

 differs only in its small size : in a moderate sized specimen (8 or 9 inches) 

 of P. edwardsii it is only about -^ inch in length. 



The proximal segments (coxocerites) of the antennae are, as in other 

 species of the genus, fused with the carapace, so that the apparently proxi- 

 mal segment is really the second or basicerite [be) ; of the coxocerites only 

 the ventral portions are left, their lateral and dorsal regions being as it 

 were squeezed out of existence by the immense development of the free 

 portions of the antennas, the articular cavities for which are thus bounded 

 above by the epimeral plates, internally by the antennulary sternum, exter- 

 nally by the anterior borders of the carapace, and only below by the coxo- 

 cerites. The fused coxocerites thus come to occupy the position of the 

 epistoma of Astacus, \ from which, however, they are at once distinguished 

 by bearing the renal apertures. Posteriorly they are continued in the 

 middle line into a projecting transverse bar (ep) which gives attachment to 

 the labrum, furnishes the antero-internal articular facets for the mandibles, 

 and apparently is the much reduced representative of the true epistoma. 



"Cat. of the Stalk- and Sessile-eyed Crustacea of N.Z., 1876. 



fCat. of the Australian Stalk- and Sessile-eyed Crustacea, 1882. 



I Huxley, " The Crayfish," p. 155. 



