BRITISH FOSSILS. 



7 



He also mentions a variation in the number of spines, but this does 

 not appear to be connected with the variation in form. We are 

 not, therefore, yet warranted in supposing that very considerable 

 difference in the appendages may be referred to sex. It is, however, 

 we think, allowable to look for independent characters in a group 

 that has no exact living representatives. Burmeister has shown us 

 that we cannot tell at what segment of a trilobite's body the tho- 

 rax really terminates, as that is determined by the position of the 

 generative pores. But, as it is extremely likely this should have 

 some external mark, we venture to suggest that the seventh or 

 eighth segment in this genus is the point where the thorax termi- 

 nates and the true abdomen begins. 



Unfortunately, among those Entomostraca most nearly allied to 

 Trilobites, we have not instances of such variation. The sexes of 

 Apus do not appear to differ much externally, and in Limulus a 

 notch in the front part of the shield of the head, and some trifling 

 differences in the feet, are all that mark the male. 



But if we turn to the Isopoda we have a direct analogy, at least 

 in one group. In Serolisy it is true, there are but slight differences 

 in the feet, the external form remaining the same. But in several 

 species of Sphoeroma, S. armata, &;c., the last or last but one ring of 

 the thorax is prolonged into a spine, very like that on the fossils ; 

 and in one species, S. diadema, if not in others, it is the characteristic 

 mark of the male ; in the female it is absent, or reduced to a mere 

 tubercle. 



Without, therefore, prematurely attempting to alter the nomen- 

 clature applied by their discoverer, I may state it as my belief, that 

 in the Remopleurides dorsospinifer may be recognized the narrow 

 form and dorsal spine of the male ; in E. laterispini/er, a mature 

 broad female form, with the eighth pair of pleurae dilated as ovige- 

 rous supports ; R. Golhii, which is intermediate in form, and desti- 

 tute of these appendages, I would suggest to be the immature 

 female ; and should further observation confirm this view, the species 

 should be re-united under the name of B. Golhii. 



Affinities. — R. longicostatus, Portlock, of which we have given the 

 characters further on, differs at a glance from each of the foregoing 

 species, not only by the great width between the eyes in front, but 

 by the narrow body axis, which is scarcely wider than the large 

 falcate pleurae. This is the only species with which they can be 

 compared ; for the R. (Caphyra) radians has the glabella strongly 

 marked by three segmental furrows on each side, and belongs to a 

 different section of the genus. R. platyceps, M'Coy, besides having 



