56 



SILURIAN TPJLOBITES. 



great terminal mucro, always equal to the length of the tail itself, and often half as 

 long again. 



There is evidently, however, a nearer connection between the var. /3. {Grindrodianus) 

 from Malvern, and the var. 7 of the last species. But while in this variety /3, which comes 

 nearest to P. caudatus, the mucrones of the head and tail are shortened, the narrow axis 

 still easily distinguishes it from the varieties of that species ; and the variety y, nexilis of the P. 

 caudatus, which has really a narrow axis, has nearly lost all trace of the processes, and has 

 scarcely a mucro at all. It is not difficult, therefore, to distinguish the two species. 



Description.-*- Pull-grown specimens must have been fully six inches long, including 

 the tail. The form is elongate, and less convex than in P. caudatus. In both varieties 

 the head is less than twice as wide as its length, but is still transverse in shape, its 

 glabella scarce one third the whole width. The glabella at its front part is parallel-sided. 

 Forehead-lobe transverse-oval, well separated from the side-lobes, which are tolerably 

 equal, — the upper one not greatly largest. Eyes large, pyramidal, and placed very near 

 the glabella, so as almost to touch it in front, and strongly curved,* reaching below to 

 the neck-furrow, and often overhanging it. 



The axis of the body and tail narrow in proportion to the sides, sometimes not one 

 fourth the width in the body-rings, and barely one fifth in the tail. The surface of the 

 body-rings is tubercular. Hinder pleurae much bent back. Fulcrum at about one half. 

 Pleural groove shallow, ending in a point beyond the fulcrum. Tail small in proportion, 

 with few (5 — 7) side-furrows, much curved and strongly interlined. Mucro abrupt, 

 linear, as long or longer than the rest of the tail. The gland-like tubercles sometimes 

 present, but often inconspicuous throughout. 



The above characters are common to the two following varieties. 



ar.miger. PI. II, figs. 19 — 21. 



Head, including the strong front mucro, much more than 

 half as long as wide. Glabella not one third the width of the 

 head. Marginal furrows strong. Head-spines as long as the 

 cheek. Mucro of tail much larger than the rest of the tail ; side- 

 furrows 7. 



I figure a remarkable specimen from Mr. Edgell's cabinet, in 

 which the spine is considerably more than twice the length of the 

 axis, and is expanded at the tip, more than is shown in our woodcut. 



One specimen of Dr. Grindrod's shows clearly the passage 

 from this form to the next variety, P. Grindrodianus, having the 

 mucro quite as long as usual, but having only five lateral ribs, and 

 tubercles on the axis and sides. Usually P. longicaudatus, var. 

 armiger, shows seven lateral ribs, and has no distinct tubercles. 

 * Much more so than in P. caudatus. 



