OROMETOPUS. 45 



the broad triangular tail, are characters which it shares with those genera, and 

 which differentiate it from other families. It is the earliest known genus of the 

 Trinucleidae, and must therefore be looked upon as the primitive form ; and the 

 conclusion is inevitable that in the later genera the absence of compound eyes and 

 the supposed marginal position of the facial suture are degenerative characters. 

 It is, indeed, no longer improbable that the ocelli which occur on the middle of 

 the cheeks in some species of Trinucleus and Ampyx may represent the normal 

 eyes, and that the lines which Salter 1 and McCoy 2 observed running from these 

 ocelli to the margins may be the fused facial sutures, as they supposed. If this 

 be so the Trinucleidae can scarcely be included in Beecher's Hypoparia. If, on the 

 other hand, as Loven, 3 Barrande, 4 and Beecher 5 contend, the true facial suture in 

 Trinucleus is marginal, then Trinucleus itself may be placed in the Hypoparia; but 

 as it is derived from an Opisthoparian ancestor, the genetic value of Beecher's 

 classification becomes extremely doubtful. 



As Mr. Raw proposes to discuss in detail the affinities of Orometopus in a 

 forthcoming paper, it is unnecessary for me to say more upon the subject here. 

 It may, however, be noted that in the young Trinuclei figured by Barrande a small 

 lobe is cut off at the internal angles of the cheeks, as in the adult Orometopus. 



Genus OROMETOPUS, Brogger. 



The type species of this genus is Orometopus elatifrons (Ang.), which was 

 described by Angelin from an imperfect cranidium, and was referred by him, with 

 doubt, to his genus Holometopus. Brogger in 1882 gave a fuller description and 

 more perfect figure; and, recognising that it did not belong to Holometopus, he 

 subsequently proposed the name Orometopus. 6 Moberg has recently given fresh 

 figures of the same form. In all these cases, with the exception of the tail doubt- 

 fully ascribed by Moberg to this species, the writers appear to have had before 

 them only the cranidium. The very fine examples from Shineton now enable us 

 to give a complete description. The specimen figured in Plate IV, fig. 8, showing 

 the cranidium and thorax, was collected by Mr. Rhodes for the Geological Survey; 

 while that represented in fig. 9, showing the tail, thorax, free cheek, and part of 

 the cranidium, was found by Mr. E. S. Cobbold, to whom I am greatly indebted 

 for the loan of the specimen. The specimens of the second species here described 



1 Quart. Journ. G-eol. Soc, vol. iii (1847), p. 251. 



3 Synopsis Sil. Foss. Ireland (1846), p. 56; Ann. & Mag. Nat, Hist., ser. 2, vol. iv (1849), p. 410. 



3 Ofv. k. Vet. Akad. F6rh. Stockholm, vol. ii (1845), p. 104. 



4 Systeme Silurien, vol. i (1852), p. 615. 



5 Arner. Journ. Sci., ser. 3, vol. xlix (1895), p. 307. 



6 JSTyt Mag. f. Naturv., vol. xxxv (1896), p. 68, note. 



7 



