146 SILURIAN TRILOBITES. 



equal in size, and the thorax of eight rings. The surface smooth, or only covered with a 

 close lineation intermixed with puncta, which does not interfere with the general smooth- 

 ness of the exterior. The head is semicircular or half-oval, or pointed, with a glabella 

 widest in front, and generally very faintly marked out, and with quite obsolete lobes.^ 

 Smooth prominent eyes ; a hypostome without rostral shield (only a vertical suture being 

 occasionally present), and a forked labrum. 



The thorax-rings convex ; the pleurae rounded, grooved, and facetted for rolUng. The 

 tail large, of many segments ; but these are not generally visible, except on the axis, and 

 in some cases not even there. 



The several subgenera are dependent chiefly on the degree of convexity, distinctness 

 of the tail-furrows, course of the facial suture in front, and the degree in which the 

 obscure glabella occupies a larger or smaller space of the head. I will proceed to give 

 these characters as briefly as I can, the route being from the Ogygides last described 

 towards the Illcenides, which we shall take up afterwards. Some of these subgenera have 

 been indicated by Goldfuss, Dekay, Dalman, Emmerich, Milne-Edwards, Burmeister, and 

 others. None of them are satisfactory, the habit having been too much overlooked, while 

 trifling differences in the glabella, &c., have been preferred to the more important 

 characters of the facial suture. Corda's arrangement is perhaps the least worthy con- 

 sideration ; and Emmerich's and Goldfuss's seem to me the best. Professor Burmeister 

 had not sufficient material for all the divisions, but his acumen detected the importance 

 of the facial suture, and his arrangement is clear. Professor M'Coy's divisions are far 

 too few. We are greatly indebted to Barrande for his laborious collection of the entire 

 history of these groups, and are sorry not to be able to adopt his own subdivisions. 

 Following Emmerich and Goldfuss, but subdividing their group of " Ogygiae " especially, 

 we have the following eight subgenera: 



1. Pti/chopi/ge, Angelin. Expanded, ovate, gently convex, or flattened, with narrow 

 axis ; short urceolate glabella, reaching more than half-way up the head, and lobeless ; 

 approximate elevated eyes ; subangular tips to pleurae ; and tail with moderately long 

 axis, and many faint ribs on axis and tail. Facial suture forming a long ogive in front. 

 Hypostome entire. Labrum shortly notched. North Europe. 



Types, A. angustifrons, Dalm. 

 A. latus, Angelin. 



2. Basilicus, Salter, 1849. Flattened and expanded forms, with rather broad axis; 

 clavate glabella reaching far up the head, with only obscure lobes, the basal pair most 

 conspicuous ; approximate depressed eyes, angular or even pointed tips to the pleurae ; 



1 Except within tbe crust, where they may be seen faintly marked out, or even strongly in some cases. 



