II BRITISH CAMBRIAN TRILOBITES. 



The gena] angles in this form are considerably produced; there is a distinct 

 margin to the head, and the glabella is bounded in front by furrows much deeper 

 and stronger than in the typical form. The last character is probably the only 

 one of importance. It is by no means unlikely that the Shiueton form may also 

 possess a margin, and that the genal angles may be produced ; but the border of 

 the head is nearly always concealed, and only doubtful indications of these 

 structures can be seen. 



All the specimens from Penmorfa show the deep furrows in front of the 

 glabella, and specimens with the same character also occur at, Arenig. Mr. 

 Fearnsides tells me that the two forms may belong to slightly different horizons. 

 They occur in material of rather different character, and apparently are never 

 found in the same block. 



S. nericiensia Wiman 1 also shows a distinct margin to the head, and the genal 

 angles are much produced, lint in the absence of any information as to the 

 character of the furrows in front of the glabella, I should hesitate to refer the 

 Penmorfa form to that species. The very small size of Winian's figured specimens 

 is remarkable. 



Horizon and Localities. — Upper Tremadoc : Penmorfa; Ceunant-y-garreg-ddu 

 and Amnodd Bwll, near Arenig. 



2. Shumardia sp. 



At Ty 'n-y-llan, near Penmorfa, both Professor Hughes and Mr. W. G. Fearnsides 

 have found remains of Shumardia in beds which appear to be of Lower Tremadoc 

 age. The specimens, though numerous, are too imperfect for identification or 

 description. They show the general form and characteristic glabella of Shwnardia ; 

 but as they occur at a lower horizon they are probably distinct from the Upper 

 Tremadoc form. 



Family Trinucleiu^;. 



The inclusion of the genus Orometopus in the Trinucleidse entails some 

 alteration in the usual definition of that family. Orometopus possesses well- 

 developed compound eyes, free cheeks of considerable size, and (in some cases at 

 least) more than six thoracic segments ; and the course of the facial suture proves 

 that the genus belongs to Beecher's Opisthoparia. Nevertheless, as the very 

 beautiful examples figured on Plate IV clearly indicate, its affinities are with 

 Trinucleus and Ampyx. The clavate glabella, the horizontal grooved pleurae, and 



1 Arkiv. f. Zool., vol. ii, no. 11 (1905), p. 14, pi. i, figs. 13—15. 



