10 



BRITISH FOSSILS. 



come, as above stated, to the conclusion that it must be considered nearly related to 

 Phacops. Barrande, in his ingenious and simple arrangement of the groups lately- 

 published,* places Cheirurus among the series which he defines as having the " plevre a 

 bourrelet ; " and certainly it is most closely allied to some genera, Spherexochus, Cyhele^ 

 &c., -which possess this character. But an inspection of our plate will show that the 

 characteristic furrow (" sillon ") of the pleurae is only shortened, not absent in this genus. 

 In several Bohemian species it is quite evident, and in the Cheirurus claviger, which Corda 

 elevates to the rank of a genus, the furrow continues along the whole length, as it does in 

 most trilobites ; and we may state generally, that we believe this character to be merely 

 a special modification, since all pleurae have the furrow, either bisecting them as in the 

 ordinary form, or so near the anterior edge as only to separate a mere line for the front or 

 fulcral portion, t In Sphcerexochus, the nearest ally of the genus we are considering, it is 

 not, perhaps, indicated at all. We think, therefore, that the other characters which we 

 regard as of more importance, viz. the structure of the eyes, and the course of the facial 

 suture, win connect Cheirurus with Phacops. But with respect to the limits of the 

 genus, we are strongly inclined to think Sphcerexochus ought not to be separated from it, 

 since such species as Ch. clavifrons of Dalman, and Ch. globosus of Barrande seem 

 exactly intermediate, having the head of Sphcerexochus and the tail of Cheirurus. How- 

 ever, if we allow the striking character of the thorax rings to guide us, we shall find it 

 agree with the habit in marking out three distinct genera already recognized, viz. — 



Eccoptochile. Corda. Cheeks scrobiculate ; pleurae 12, furrowed; hypostome with 

 lateral furrows : 



Cheirurus. Bey rich. Cheeks scrobiculate ; pleurae 11, nodulated; hypostome with 

 lateral furrows : 



Sphcerexochus. Bey rich. Cheeks not scrobiculate; pleurae 11, simple, rounded; hy- 

 postome without lateral furrows : 



And the species which have globose glabella, but still have the 1 1 nodular pleurae, will 

 remain in Cheirurus, not in Sphcerexochus, just as we find this variation in the glabella of 

 Phacops, while the characters of the thorax remain the same. 



The genus is Silurian and Devonian ; it does not rise into the Mountain Limestone. 



Other British species of Cheirtjrtjs. 

 Section Ckotalocephalus. 

 Glabella furrows continuous across, all strong and distinct. 



1. C. articulatus? {^Calym. articulata, Miinst. Beitr. Heft, 3. pi. 5. fig. 7.?] Ch. Stern- 

 bergii (Miinst), Phillips, Pal. Foss., fig. 247. 



I do not venture to characterize this species from the imperfect fragments we possess. 

 The glabella is long, narrow, and scarcely clavate forwards, and not very convex. The 

 upper and middle glabella furrows are very strong, and go right across, and the basal 

 lobes are narrow, triangular, transverse, and they nearly meet in the middle of the glabella, 

 leaving but a small space between. 



The latter character I suspect to have been much exaggerated by Miinster, in the figure 

 above quoted, who has represented the basal lobes as forming one transverse piece. Our 

 rare British fossil is certainly more like this figure than the C. Sternbergii of the same 

 author, in which the furrows are partially obliterated in the middle, and the glabella is 

 broader. 



Locality.— l^QxiGn, S. Devon (Phillips) ; Newton Bushell, in Devonian limestone. Pre- 

 sented by E. A. C. Austen, Esq. 



* Systeme Silurien de la Boheme, 1853. Also Ann. and Mag. Nat. History, Sept. 1850. 

 ■j- This narrow line may certainly be seen in Acidaspis and Cybele ; in Bronteus it 

 seems to have completely vanished. 



