ON FOSSIL CRUSTACEA. 305 



By the kindness of Professor Owen I am permitted to add three plates from 

 his Memoir on the modern American King crab to Uhistrate my ' Monograph 

 on Fossil LimulL' I have also introduced (from Dr, Packard's and Dr. 

 Dohrn's works) figures of the larval stages of Limulus pohipliemus ; and 

 from that of Barrande figures of the larval forms of certain Trilobites, the de- 

 velopment of which he has traced often (as in the case of Sao hirsiUa) through 

 more than twenty stages. 



Having read carefully the arguments of Dr. Dohrn, and subsequently 

 the views of Dr. Packard, the elaborate papers on the anatomy of Lumdtis 

 by Alphonse Milne-Edwards and Prof. Owen, I find nothing in these several 

 memoirs to lead mo to distrust the conclusion at which I had arrived in 

 1866 (see Brit. Assoc. Reports, Nottingham, and Quart. Journ. Geol. Soc. 

 1867, vol. xxiii. p. 28) as to the correctness of associating the Eurtpterida 

 and XiPHOsuRA under the Order Merostomata, but much to confirm and 

 strengthen that conclusion. 



Prof. Owen fully concurs in my general views of the Merostomata, as 

 an order, although he differs from me in some minor points in reference to 

 the structure of Limulus. 



For example, he considers the anterior shield, as I do, to be the cephalon, 

 merely proposing for it the term cephaletron* ; whilst for the posterior shield 

 (which I demonstrated in 1866 to be the conjoined thorax and abdomen) he 

 gives the name thoracetron ; and to the telsou, or tail-spine, he apphed Mr. 

 Spence Bate's name of "pleon." 



There can be no objection to the term " cephaletron," as proposed by Prof. 

 Owen, for the head in Crustacea, in contradistinction to that highly special- 

 ized division of the body, the " head " in the Vertehrata ; but I think I have 

 shown good grounds (in the paper above referred to) for assuming that the 

 'posterior shield is not merely the thorax (or " thoracetron" of Owen), but the 

 combined thoracic and abdominal segments, as attested by the larval or em- 

 bryonal stages of Limulm, and by the fossil forms of the Coal-measures and 

 of the Silui'ian. 



I venture also to demur to Spence Bate's term " pleon " being restricted to the 

 tail-spine in Limulus, because it is calculated, if so used, to cause considerable 

 confusion. The term " pleon," as applied to the Crustacea by its author, 

 includes the last seven segments of the body, of which the telson (if reckoned 

 at all as being a segment) can only be assumed to be the ultimate joint of 

 the series. 



The view propounded by Prof. Owen — that the great caudal spine in Limulus 

 represents, either by itself or possibly with the hindmost segments of the 

 "thoracetron" (Owen), the "pleon" of Spence Bate (or in other words the 

 last seven (or abdominal) segments usually seen in other Crustacea) — is based 

 on his examination of the innervation of the tail-spine. From its dissection 

 he finds that the bifid continuation of the great neural axis is divided within 

 the triangular tail-sheath into a double plexus of fine nerves resembling the 

 Cauda equina of anthropotomy. In this fasciculus of ncrvc-threads the author 

 traces nine nerve-filaments, four ventral and four dorsal, the ninth being the 

 continuation of the bifid neural axis. From this he concludes that the tail- 

 spine may indicate as many as four coalesced segments, which with the three 

 posterior coalesced apodal segments of the " thoracetron "' would account for 

 the missing abdominal series, or the "pleon" of Spence Bate. 



* From Ke^aXr/, the head, and rjrpov, apart of the aklomen, in allusion to the fact that 

 "a part of such cavity is associated with the 'head' in the first division of the King 

 crab's body, and with the ' thorax ' in the second division." (Owen, op. cit. p. 463.) 



1873.' X 



