PHACOPS. 51 



Thorax with the central lobe not separated by any strong furrow from the pleurae, 

 which in English specimens are half as wide again as the axis ; the axis is a little fusi- 

 form, not broader than the base of the glabella, and either lias the rings smooth, tubercled 

 at their outer edges, or with two tubercles on the central prominent part. All these varia- 

 tions occur on the same thorax. The front pleurae are flat for the first half, then gently 

 curved down, and a little backwards, at the fulcrum ; they are divided by a moderately 

 strong curved furrow for most of the length, and their edge sharpened anteriorly (the flat, 

 surface not crossing the furrow, as in Acaste), for rolling or bending. The extremities are 

 truncate, and pointed at the hinder angle ; posterior pleurae with the fulcrum at less than 

 half, much more curved back and strongly pointed, overlapping the upper corners of the 

 tail. Whole surface of thorax finely granular. 



Tail with the sides meeting below at nearly a right angle, and produced more or less 

 to a sharp point beyond this ; axis not much raised, and gently tapering, ending a little 

 abruptly at some distance from the point ; it has eleven or twelve ribs, which are occasion- 

 ally tubercular, like the thorax, sometimes much so ; the sides have six, seven, or some- 

 times eight narrow ribs, strongly arched down at their ends, where they abut suddenly on 

 the smooth margin ; each is sharply defined behind, and duplicated on its forward edge 

 by a fine rib along its whole length. The number of ribs appears to be as great in 

 the young trilobites as in the adult form. 



The above description, extracted from the Decade, will serve for the general account 

 of the species. I distinguish the following varieties more particularly. 



VaR. a, VULGARIS. PI. Ill, figS. 4 17. 



Form narrow-ovate. Head broad, rounded, or slightly angulated only in front. Glabella 

 as broad as the cheeks. Tail truly triangular, simply pointed, scarcely at all acuminate. 



There is no essential difference becween any of the specimens of this common variety. 

 All have the rounded or arched front margin (not at all produced) to the head ; the 

 glabella at its widest part much more than one third the whole width of the head. The 

 length of the head as near as possible half the width of the same ; the axis broad j the tail 

 shorter than the width, and simply pointed, with a very short mucro, or none at all. 



They vary a little among themselves in the following particulars. Some have the 

 glabella more strongly marked out, and more decidedly narrowed behind. Some have 

 the glabella-furrows a little deeper. The eyes are larger in some (as in the varieties of 

 P. Doivningia, before described), and more prominent ; but I have elsewhere referred these 

 differences to sex. The male had probably more prominent eyes. 



Some specimens have the characteristic tubercles on the sides of the axis, and the 

 central ones on the tail much stronger than others. We have figured these tubercular 

 varieties in PI. Ill, figs. 15 — 17. As these seem to be chiefly characteristic in the young 

 state, and are much more obscure in the full-grown animal, I will here notice them par- 

 ticularly as they occur in the young and older states. 



