﻿198 BRITISH FOSSIL CRUSTACEA. 



" Seeing the relations of these nerves as continuations of the neural axis, and the like 

 relation of the artery of the spine to the dorsal vessel, I long ago (writes Prof. Owen) 

 concluded the spine itself to be a continuation of the series of body segments, to be 

 serially homologous therewith, and not with their 'appendages.' The coccygeal style of 

 the Frog's endo-skeleton 1 is analogous to the tail-spine of the King-crab's exoskeleton. 

 The antecedent part {abdomen, H. W.) wherewith the spine is articulated has no limbs. 

 Is it also part of the 'pleon ' ? {abdomen, H. W.), and does the post-ganglionic part of 

 the neural axis indicate the extent of such part ? " 



[In an interesting conversation with the celebrated carcinologist Professor Henry 

 Milne-Edwards in October, 1873, shortly after the publication of Professor Owen's 

 Memoir from which I have so largely quoted, we discussed the question of the homo- 

 logies of the tail-spine or ' telson ' in Limuhs, and he then frankly expressed his opinion 

 that the nerve-structure described by Professor Owen as characterising the tail-spine of 

 Limulus warranted the conclusion of that distinguished anatomist, and that it no doubt 

 confirms his view that it represents the posterior segments of the rudimentary abdomen. 



Although this conclusion differs from that expressed by many other writers, as well 

 as by myself, who had considered the ' telson ' or tail-spine as a median appendage 

 and not a true segment (in which view its development at a later period in the life of 

 the young King-crab seemed to lend confirmation), nevertheless I felt bound to reconsider 

 the facts most carefully, and also to ascertain, if possible, whether there was any evidence 

 which might be adduced from fossil forms bearing upon this question pro or con. 



The evidence afforded by Hemiaspis limuloides (see Part IV of this Monograph, pp. 

 174—177, woodcut, fig. 64, and PL XXX, figs. 1 and 2) in favour of Prof. Owen's 

 view of the telson in Limulus had not escaped the attention of that eminent observer. 



At p. 176 I wrote as follows : In Hemiaspis "the abdomen consists of only three 

 segments, each 2 lines in breadth and 1^ line in depth. The first has no epimera, and 

 appears to move freely at its articulation with the sixth thoracic segment. The second 

 and third segments have small epimeral pieces, which are bilobed, with the posterior lobe 

 more pointed. A line of small tubercles runs down the centre of these three joints, which 

 are somewhat raised at their articular borders. 



"The smallness of the abdomen, and its reduction from the assumed normal 

 number of six segments to three, seems to indicate a form by which, with the help of 

 others, we may bridge over the interval that has hitherto existed between these two 

 groups, the Hurypterida and the Xiphosura" (see woodcut, fig. 72, p. 200). 



In his Monograph on the anatomy of the American King-crab Prof. Owen 

 writes (p. 500) : 



" Segments indicated by the nerve-pairs, but concealed or suppressed by the crust at 

 the base of the tail-spine in Limulus, were realised in Hemiaspis limuloides, H. Woodw. 



" The progress from the general to the special, from vegetative repetition to 

 1 Owen, 'Anatomy of Vertebrates,' vol. i, p. 49, fig. 44 c. 



