BRITISH FOSSILS. 



3 



less than half a circle, its entire width being but little more tlian 

 that of the glabella ; the axis is short conical, occupying one third 

 its width, with one distinct ring, another more obscure, and a ter- 

 minal joint ; sides with one distinct upper furrow, which does 

 not reach the margin. 



All the prominent parts of the surface of the body are rough 

 with small tubercJes ; but these are by far most evident on the 

 glabella, cheeks, and neck segment ; they are wider apart than 

 their own diameter, and pretty regular in size. 



Variations. — The forehead portion of the glabella in our figs. 3 

 and 8* is much smaller and less inflated than usual, giving the glabella 

 a parabolic instead of sub-rectangular form ; and the same variation 

 is less conspicuous in fig. 7- In other respects they seem to be 

 identical. Some Lower Silurian specimens have the space in front 

 of the glabella a little wider than in those from Dudley, but even 

 in Dudley specimens the anterior margin is sometimes narrower 

 than this space and sometimes broader. 



Sex. — Under the genus Remopleurides, described further on, at 

 plate 8, the possible indication of sex by certain ornaments or appen- 

 dages to the dorsal surface is adverted to. Of the small nmnber 

 of this species hitherto examined, we have met with no individuals 

 destitute of the spine at the 6th segment, and it is therefore 

 quite possible that it may be characteristic of the species, and not of 

 one of its sexes. In the collection of Mr. Fletcher, of Dudley, one 

 specimen (fig. 7 ) has the spine nearly double the ordinary length, or 

 twice the length of the five anterior thorax rings. And this varia- 

 tion, which we can hardly help regarding as indicative of the male, 

 is accompanied by a less inflated glabella, the basal lobes being set 

 more widely apart, as above mentioned, and by a somewhat more 

 pointed form of the head. In G. Burmeisteri, the large Bohemian 

 species, the curved dorsal spine always occurs on the same 6th seg- 

 ment, and is always long.* But it is at least worthy of remark 

 that the possession of such a dorsal spine is characteristic of the 

 male of some of the CymotJioadw, a group of Isopod Crustacea 

 very analagous, though probably not closely allied to, the Trilobites. 

 In the genus Sphceroma, the male of one species, S. diademo-, is 

 characterized by the presence of a spine very much like that of 

 Cyphaspis, and occurring too on the 6th thoracic segment ; in the 



* In Encrinurus punctatus, described above, pi. 4, such spines, but much shorter, occur 

 on the 7th and 10th segment ; and there are certain trilobites, Sao hirsuia and Bionteus 

 spinifer, Barrande, for example, that have a prominent spine on every thorax ring : so that 

 we must estimate tliis character at no more than its proper value. 



