PHACOPS. 27 



and the tail has the side-furrows more than usually obsolete. Mr. Ketley's specimen, 

 (fig. 27) has the front glabella-furrows strong, and the pleurae flattened. The tail is of 

 the usual type, but the side-furrows are obscured. 



Var. y, inflatus. PI. II, figs. 30, 31 (32, 33 ?). 



Fig. 30 shows a remarkable variety. While it retains most of the characters of the 

 ordinary form, it nevertheless puts on a very different aspect. The glabella is greatly 

 swollen, so as to be very convex, instead of flattened, in front. All the furrows are indeed 

 distinct and in their proper situations, but from the inflation of the glabella they appear 

 crowded. The eyes are small. Dr. Grindrod's cabinet and the Museum of Practical 

 Geology are the only collections which I know to contain this variety. Probably fig. 31, 

 a Ledbury specimen, belongs to var. j3. It has the upper glabella-furrows all but obsolete. 



On the other hand, figs. 32, 33, also from Ledbury, show all the furrows of the head 

 and tail stronger than usual. These are casts of the interior, and the thickening of 

 all the internal ridges is a constant character in Trilobites. 



Var. ? S, spiNosus. Woodcut, fig. 7. FlG - 7 - 



Agrees with the ordinary variety a in the glabella, but has short 

 head-spines ! Only a single specimen is known ; it is in Mr. Edgell's 

 collection. With it, however, occurs a pointed and strongly furrowed PhacopsDownmgia>,xz,T. 

 tail, which may belong to it, and would mark it as a very distinct form; LudlbTrock, Ludb " 

 possibly a species. 



But to pass on to a marked and definite variety, which has not yet received a 

 name, and which would by many be considered a distinct species. I propose, however, 

 only to term it — 



Variety or Sub-species 1 e, constrictus. 



Phacops constrictus. PI. II, figs. 13 — 16. 



Minor, fronte convexiori rotundato haud angulato, oculis magnitudine variis, prope 

 glabellam positis. Cauda rotundata, sulcis obscuris. 



1 I am much disposed to follow the plan adopted by some eminent botanists, and to make a distinction 

 between the occasional variations in form and structure which are usually termed varieties, and tbose 

 more permanent and well-defined groups, which are designated sub-species. Regarding these latter, there 

 will always be differences of opinion as to whether they should receive separate specific names, and it seems 

 the most convenient plan to describe them as distinct forms, which may be considered either species or 

 varieties by the student, while their supposed relation to the parent species is indicated by their being 

 grouped under it. We do not yet know what the limits of species and varieties are, and probably there is 

 no real line to be drawn ; but all truly distinct forms should receive attention, and, for the purpose of the 

 geologist especially, a marked variety is as useful as a species. 



