OF THE SPECIES. 79 



Trilohifes having the poiccr of rolling t/icnwelves into a ball, with the axis of the body contracted 

 posteriorh/, the shell (/ranulated, and generally more than ten body rings* Calymenid^E. 



This natural section I formerly subdivided according to the number of body rings into 

 three genera, having respectively thirteen joints, eleven joints, and ten joints. There have 

 since, howe\er, been found forms which render this method of grouping unadvisable, and I 

 now prefer taking the course of the facial suture as the basis of arrangement. 



Trilobites capable of rolling themselves up, and whose facial suture terminates exactly in 

 the angles of the cephalic shield. 



It appears that there arc but two genera that can be included in tiiis subdivision, and 

 for these I retain the names of Calymene and Ilomalonotus. They are distinguished from one 

 another by the cephalic shield, which in Calymene is furnished with a reflexed margin, over 

 which the anterior extremity of the facial line extends ; whilst a flat expanded margin is 

 found in Homalonotus, the anterior ends of the facial line meeting in the centre of the 

 margin of the forehead, before the glabella. Other differences accompany this principal 

 one, and justify the separation of the genera. 



Genus 11. — Calymene, Brongniart,f (Amphion and Zethus, Pander). 



Cephalic shield semilunate, rather strongly convex, furnished with a margin which is 

 reflected all round, the largest and anterior division usually broken ofl'. The glabella, 

 which is always rather contracted towards the anterior part, has a high, much reflected 

 margin of articulation, and besides this always two or three sulcations at each side, by 

 which it is divided into three or four lobes. If only two sulcations are present, then it is 

 the anterior one which is wanting. The hindermost lobe of each side is the largest, and 

 considerably arched ; the second from the posterior extremity is next in point of magnitude ; 

 the third is usually the smallest, and frequently very imperfectly separated from the last or 

 anterior one, especially in the most common species, C. Blumenbachii. The cheeks extend 

 by the side of the glabella as independent convex plates, and bear strongly projecting but 

 not very large eyes, the horny membrane of which is either wanting or pressed in. They 

 are placed sometimes on the centre (C Blumenbachii), sometimes on the anterior half of 



* As additional characteristics, it may be stated that the members of tliis group ahvays exhibit a 

 highly arched caudal axis, with distinct rings and radiated lateral furrows proceecUng from it. These 

 furrows are absent, if not in both, at least iu the second group of the second division. The glabella 

 likewise has (with only two exceptions) lateral fuiTows and lobes. 



t If Murcbison's figure of C. variolaris {Sil. Syst. PI. XIV, Fig. 1) be correct, the animal had 

 thirteen body rings, and l)elongs to this division. Preceding authors, as Parkinson (Org. Rem. iii, PI. 

 XVII, Fig. 16) and Brongniart {Cr.fos. PI. I, Fig. 3,) enumerate only eleven, indicating an affinity 

 with Phacops. 



