ISOTELUS. 165 



furrow, so characteristic of this subgenus ; the labrum is subquadrate, broader above and 

 narrower towards the truncate tip. It appears not to be at all bilobed (unless it be 

 imperfect in our specimen ; but if it be not, A. affinis does not belong to Isotelus, but must 

 form the type of an entirely new subgenus). It resembles some of the Swedish species 

 of Cryptonymus, but the course of the facial suture and the indistinct glabella will 

 distinguish it from these ; while from the species of Isotelus the well-marked-out axis of 

 the caudal shield will easily separate this primordial species. 



Locality. — Upper Tremadoc Slate. Pen-y-clogwyn, S. of Portmadoc, in flinty 

 slate, much compressed by cleavage ; at Garth, Penrhyn ; and near Llanerch, by the roadside 

 towards Treflys. Prof. Sedgwick's specimen came from the slopes above Tremadoc ; and 

 I have gathered this species at Tyddyn-dicwm, on the Tremadoc mountain-side. 



[This and the following are the earliest true species of Asaphus known ; if, as above 

 said, the labrum do not betray an affinity with Oyyyia.'] 



AsAPHUs (Isotelus) Homfrayi, Salter. PI. XXIV, figs. 6 — 12. 



Asaphus Homfkayi, Salter. App., Ramsay Geol. N. Wales, Mem. Geol, Surv., vol. iii, 



p. 311, pi. viii, figs. 11—14, 1866. 



A. (Isot.) longe ovatus, lente convewus, capita ante subanyulato, spiculis brevihus. 

 Sutura facialis intramaryinalis. Oculi submedii, parvi. Axis caudce lonyus^ ad apicem 

 prominulus. 



This is a more common species than the last, and is, indeed, not infrequent in the 

 Tremadoc district. It is conspicuous too, being three inches long and one and a half 

 broad. Compressed as our fine specimen from Mr. Homfray's cabinet is, it measures 

 three and one third inches by three quarters of an inch. The Museum of Practical 

 Geology has a fine series, presented by the same gentleman, to whom geological science 

 is indebted for much hard work in the Tremadoc district. 



The head is more than a third of the whole length, and longer than the thorax, 

 which, in its turn, is longer than the caudal shield. The head is semi-oval, rather 

 pointed in front, and has very short posterior spines ; it is broadly depressed round the 

 margin. The glabellar portion is scarcely marked out ; the eyes are placed nearly half 

 way up the head ; they are small (two lines long), the facial sutures curving out boldly 

 beneath them, and cutting the posterior margin more than half-way out from the axis. 

 Above the eye they form a narrow ogive, and nearly follow the front margin. On the 

 under-side of the head the vertical furrow on the epistome shows distinctly through the 

 cast. The labrum is imperfect, but exhibits a strong marginal groove and two small 

 lateral furrows. 



The body-rings have the axis as broad as the sides, and moderately convex. The 



