64 SILURIAN TRILOBITES. 



pletety across, and with an oblong shape of glabella, the aberrant species having it oval, 

 and some even globular, as we shall see further on. 



It was first described as a Paradoxides by Murchison, who only knew the caudal 

 portion ; the head was figured about the same time by Hisinger, under the name C. speciosa 

 of Dalraan, an identification which there is now every reason to doubt, as the synonymy 

 has been cleared up by the labours of Angelin, to whom we should be still more indebted 

 were his descriptions less brief. As I have endeavoured in the Decades of the Survey 1 to 

 show the history of the species, I need not go over all that ground again, and I retain the 

 name C. bimucronatus, as already well known and adopted by several authors ; indeed, it 

 appears to have been the earliest. 



Var. a, bimucronatus. PL' VI. figs. 15, 16, 17. Cauda mucronibus posticis august is, 



centrali nullo. Decade 7 ; Geol. Survey, pi. ii, figs. 4, 5, 6. 



Var. ft centralis. PI. VI, figs. 9, 10, 11, 12, 13, 14, 18. Cauda mucronibus sub- 



cequalibm, centrali brevi distincto. Mem. Geol. Surv., 1. c, fig. 

 7 ; Decade 7, pi. ii, fig. 16. 



The following is the description given in the Decade, with some modifications, and 

 the omission of details. 



One of the larger Trilobites ; it measures occasionally nine (not twelve) inches, but 

 probably not more. The more perfect specimens found at Dudley are not above two or 

 three inches long, those from Malvern are larger, and those from the Caradoc limestones 

 of South Wales and Ireland the largest of all. Length to breadth as 3 to 2 ; the 

 head occupies fully one third the length, and is a little broader than the body. General 

 form moderately convex and oblong, but narrowed suddenly towards the posterior end ; 

 the sides of the thorax and tail deeply serrated by the projecting ends of the segments. 

 The animal is sometimes found half coiled up ; the pointed ends of the pleura? closing 

 together, and overlapping each other. 



Head rather more than a semicircle, the obtuse front projecting; glabella gently 

 convex, equal in breadth at the base to the cheeks, above considerably broader, marked 

 with three strong furrows on each side, besides the neck-furrow, the lowest being directed 

 obliquely downwards, and joining the neck-furrow before reaching the middle ; it thus 

 encloses a spherical triangle as a basal lobe. In older specimens this lobe is somewhat 

 squarer, and the furrow more curved. The other furrows curve but little downward, and 

 are variable in length, but usually extend more than one third across the glabella on each 

 side. The furrows on the glabella, as well as the axal furrows, are sharp, but not broad 

 or deep exteriorly, although they are so on casts of the inner surface. Porehead-lobe of 



1 Decade 7, ' Geol. Surv.,' plate ii. 



