BRITISH FOSSILS. O 



above. Round the head a strong furrow separates the broad margin, 

 stops at the base of the spines, and there nearly meets the equally 

 strong neck furrow, which is arched down at its end. The margin in 

 front more or less produced into an obtuse point. 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 fusiform, not broader than the base of the glabella, and either 

 has the rings smooth, tubercled at their outer edges, or with two tuber- 

 cles on the central prominent part : all these variations occur on the 

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

 curved down, and a little back, at the obscure 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 

 true Phacops) for bending. The extremities are truncate, and pointed at 

 the hinder corner ; 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 11 or 12 ribs, which are accidentally tubercular, like the 

 thorax, sometimes much so ; the sides have six, seven, or sometimes 

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

 suddenly on the smooth margin ; each rib is sharply defined behind, and 

 duplicated on its forward edge by a fine rib, running parallel to it 

 along its whole length. The number of the ribs on the tail appears to 

 be as great in the young animal, judging from the beautiful specimen, 

 Fig. 7, from Mr. J. Gray's collection. 



Variations. The tail margin varies much in width. In fig. 15 it is 

 very narrow, and the apex little produced. Most of the specimens of 

 var. a, in shale or mud (Fig. 11), have the axis of the head and tail 

 narrower than those in limestone (Fig. 12), and the tail itself broader 

 and more mucronate. From Ludlow we have specimens of this form, 

 with all degrees of attenuation in the mucro. The larger head of 

 var. /3 (Fig. 14), has the outer margin more expanded than in any lime- 

 stone one yet seen. 



Sex. Some differences are very observable between individuals of 

 this species, as well as of other species and genera, which are possibly 

 referable to sex. We find the male in insects distinguished by large and 

 prominent eyes, extra length of spines, and narrowness or depression of 

 the body ; the latter character is sexual in the Crustacea. It is natural 

 to look for some sexual distinctions in animals with such marked 

 forms as trilobites have, but an examination of Scrolls, the external 



