42 



attached to the Limuline cephaletron. As in Limulus, moreover, a partially coalesced 

 pair of opercular plates extended backward in Pterygotus from the under and hinder 

 border, more or less concealing the underparts of the two anterior segments of the , 

 thoracetron (fig, 14, b). 



The foremost division of the body (a), in both Slimonia (fig. 15) and Fterygotus, is 

 composed of fewer segments than in Limulus. The next division of the body, b, in Eury- 

 pterids, includes a greater number of segments ; and the broadest of these but little exceed 

 in that dimension the coalesced cephaletral segments. The pleonal ones, transitorily 

 manifested at the basal part of the telson in Limulus, retain their individuality in Eury- 

 pterids, so that the distinction between thoracetron, b, and pleon, c, is arbitrary, and 

 only the telsic termination of the third division of the body is definable. Accordingly, 

 the whole body of the extinct Merostomes is longer and narrower, exhibiting less of 

 concentration and more of irrelative repetition, than in the existing Limuline form. 

 If shaj)e and relative size afi'ect so little the conclusion above supported of the homo- 

 logy of the cephaletron in Xiphosures and Eurypterids, much 

 weight cannot be attached to the difference of form and propor- 

 tions of the ' pleon ' or ' telson ' in the same question, especially 

 with present knowledge of the intermediate modifications of this 

 division of the body, as seen in Eurypterus (fig. 13)* and Stylon- 

 tirus (fig. 16). As the cephaletron of Limulus includes more 

 segments and appendages than does that of Fterygotus, so like- 

 wise may the pleon of Fterygotus as compared with that of Li- 

 mulm. The excess of segments of the thoracetron in Eurypterids 

 (which excess H. Woodward is disposed to refer to another divi- 

 sion of the body, which he terms ' abdomen ') may be among 

 those of which embryologists of Limulus believe themselves to 

 have seen traces in its budding tail-spine. However that may 

 be, or be accepted, the pleon or telson in all Merostomes is ter- 

 minally pointed or spinous, and would help in the movements of 

 Styhneurus Logani, Wd. the animal much in the same way as Lloyd and Lockyer have 

 observed it to act in Limulus. 



That this tail-spine (pleon and telson) is a serial homologue, reduced and simplified, 

 of the segments, and not in the category of limbs or other mere appendages, the modifi- 

 cations thereof in some of the extinct allies and predecessors of Limulus give evidence of 

 weight. The argument for its appendicular grade, from time, " that it is developed sub- 

 sequently to the other segments," can only apply on the assumption or supposition that 

 all true segments or ' somites ' of a Crustacean are simultaneously developed. The state- 

 ment that the ' tail-spine ' is developed not only subsequently to, but " from the dorsal 

 surface t " only of the body, has but the value of an unsupported assertion. If the attach- 



* Nieszkowski, " Der Euryptcrus remipes aus den obersilurischen Schichten der lasel CEsel." Archiv. fiir die 

 Naturkunde Liv- Ehst- u. Kurlands, Erste Serie, Bd. ii. tab. i. fig. 1" ; quoted by Anton Dohm, he. cit. p. 640, Taf. 

 xiv. fig. 21. H. Woodward, Eunjpterus hrodiei, from Perton, ' Quarterly Journal of the Geological Society,' March 

 1871, p. 261, fig. 1. t Prof. Huxley, in 'Medical Times and Gazette,' 1857. 



