OF THE SPECIES. 101 



GeiiKs 16. — Archegonus, (Phillipsia and Griffitiiidrs, Portl.) 



The cephalic shield is rather large, parabolic, moderately convex, with a niarcrin but 

 slightly thickened, the glabella is evident, but not so distinct as in the former genera, 

 its form is different, its sections and lobes not very depressed. The postci-ior angles of 

 the cephalic shield are either pointed or obtuse. 



The facial suture commences at the anterior margin of the cci)halic shield, turns thence 

 somewhat inwards towards the eye, forms the covering plate, and extends in a curve to 

 the posterior margin, which it intersects near the middle. 



The eyes are small, particularly low, but frequently long, lunate, and of a finely trans- 

 parent lattice-work. 



The axis of the body consists of nine short, highly arched rings, and is only very slightly 

 narrowed towards the posterior part ; the lateral lobes are as broad as the axis, and have 

 only a distinct oblique furrow. 



The caudal shield is but very little smaller than the cephalic, its form parabolic ; 

 it is highly arched at the axis, and less so at the sides ; the axis is distinctly articulated, and 

 consists of twelve or more joints ; the sides are furnished with radiated furrows. 



The surface of the shell is granulated in most of the species, but in some it is finely 

 lineated. 



Localifi/. — The carboniferous limestone and other contemporaneous beds. 



Remark. — WLeu I first described this genus (which I did contemporaneously with Portlock), 

 I knew accurately only one species, and owing to the shell of this species possessing a lineated surface, 

 I placed Archegonus in the next group of Trilohites. This genus, however, has become better known 

 since, and it has been proved that the shell of most species presents a granulated siu-face. I have 

 therefore been obliged to alter its systematic position. The occurrence of two different kinds of 

 mai'kings in the same genus is a remai'kable circumstance, but less enigmatical in this instance when 

 we take into consideration that this genus represents the last type of the Trilobites, and therefore 

 naturally would bring together characters which hitherto had been distributed over different contemporary 

 genera. A similar combination takes place filso in Brontcus. 



The species may probably be grouped best in the following manner : 



I. Those in which the glabella has three lateral furrows, which obliquely extend 

 towards the posterior part, and become gradually larger; the posterior and largest separates 

 a lobe which is more strongly arched and more projecting ; and the elongated eyes corre- 

 spond with the latter in position. The posterior angles of the cephalic shield are elongated 

 in a granulated manner. — Phillipsia, Portlock. 



A. The glabella not broader towards the anterior part, but of the same breadth, 

 and commencing from the eyes, afterwards parabolically rounded. The shell has a tuber- 

 cular granulation, particularly at the axis. 



To this belong the species, Phill. Kcllii, Portl. {Rep. of Geol. etc., 307, Table II, Fig. 1) ; 

 Pliill. oniata, Portl. (/. c. 307, Fig. 2) ; the figure in the same work marked Figs. 4. 10. 11. 12 ; 

 the caudal shield in Brongniart's Crust, foss. Table IV, Fig. 12; Phill. gemmulifera, 

 De Koninck, {Mem. de VAcad. Poy. de JBruxelles, torn, xiv. Fig. 3 ; Ej. Anim./oss. de la Belyiq. 

 603. 4, Table LlII, Fig. 3.) Perhaps we may also include among this group Amph. 



