184 H. HICKS ON THE SUCCESSION OF 



Illcenus Htjghesii, n. sp. PL. IX. fig. 7. 



The first specimen of this species was discovered some years ago 

 by Prof. T. M'K. Hughes, at Llanrian, near Aber-eiddy ; and this 

 specimen, along with another in my possession from the Llanvirn 

 quarry, was figured by the late Mr. Salter in his memoir (Palseonto- 

 graphical Soc. vol. xx.) as Illcenus perovalis. Now, however, that 

 further and more perfect specimens have been found, there can be 

 no doubt that the species has been incorrectly determined. They 

 indicate a species allied to, but yet quite distinct from, the Illcenus 

 perovalis of Murchison ; I have therefore named the species after its 

 discoverer. Mr. Salter's description unfortunately pertains rather to 

 this species than to Illcenus perovalis, from his having been misled 

 by the specimens from near Aber-eiddy: and he has also placed 

 its position in the series too high ; for as yet it has only been found 

 in the Upper Arenig group ; and it has not been found associated, as 

 there stated, with Didymograptus Murchisoni. 



Description. — A very broad oval species, from 2| to 3 inches long 

 and about 2 inches broad. Head semicircular, moderately convex, 

 and occupying rather more than a third of the whole length ; the 

 glabella is divided from the cheeks at the lower part by tolerably 

 deep axial furrows as far as the base of the little oval glands. The 

 eyes are separated from the glabella by a space equal to about two 

 thirds of the width of the latter ; they are of moderate size, and are 

 situated rather near to the neck-margin. Axis of the thorax strongly 

 raised, and separated from the lateral lobes by deep axial furrows ; 

 pleurse longer than the axis, sharply pointed, and with the fulcrum 

 about midway. Tail semicircular, with the front edge slightly 

 arched ; axis narrow and conical, and reaching about half the length ; 

 side-lobes moderately convex, and bounded on the outside by a wide 

 margin. 



This species is a much broader form than /. perovalis, has a more 

 distinct glabella, and the eye is larger and placed nearer the neck- 

 furrow. The position given to the eye in Mr. Salter's figures is not 

 correct for this species, though apparently so for /. perovalis. 



Locality and Formation. Llanvirn Quarry : Upper Arenig. 



Ill^nopsis? acuticatjdata, n. sp. PI. IX. fig. 5. 



The genus Illcenopsis was instituted by Mr. Salter for a form sup- 

 posed to be intermediate between Illcenus and Asaphus, and which 

 had then been found only in the Arenig rocks of Shelve. The 

 generic characters given by him are : — " Eyes remote, forward ; 

 glabella-furrows reaching the front ; head not gibbous, only convex ; 

 pleurae grooved." Our species approaches nearer to this genus than 

 to any other ; and I place it under it at present, though with some 

 doubt. Ours also occurs in the Arenig rocks and at very nearly the 

 same horizon as Mr. Salter's species. 



Description. — Ovate in shape, about 1 J inch long and ^ of an 

 inch wide. Head semicircular, very convex, and nearly equal to 



