146 BRITISH PALAZOZOIC FOSSILS. [Crustacea 
TRINUCLEUS ? RADIATUS (Murch.) 
Ref.—Murch. Sil. Syst. Pl. 23. fig. 5. 
Sp. Ch.—Cephalic shield rotundato-quadrate, little more than twice as wide as long; margin very 
wide, at least as deep as the cheeks at the sides of the front; anterior margin wide, concave; anterior 
lateral angles obtusely rounded, sides nearly straight, generally inclining slightly inwards towards the posterior 
lateral angles, from each of which a short slender spine extends outwards and backwards at an obtuse 
angle with the sides; about four rows of punctures in front, more numerous at the sides, connected in 
lines by strong radiating furrows; glabella ovate, gibbous, cheeks little wider than high, convex. Usual 
length of cephalic shield seven lines. 
This species is easily recognised by the great width and square form of the radiated border to the 
head, but though I have seen many specimens none have shewn me the glabella or cheeks in perfect 
preservation; I am consequently uncertain whether they have the characters of a true Zrinucleus or of 
a Tretaspis. 
Position and Locality—Common in the slates of Dinas Mowddy, Merionethshire; N. of Tremadoe, 
Merionethshire. 
Genus. TRETASPIS (M’Coy). 
Ref.—M°Coy, Ann. Nat. Hist. 2nd Series, Vol. IV. 
Gen. Char.—General characters of Trinucleus, but having but five body-rings, the base of the glabella 
having two short segmental furrows on each side, and the cheeks being traversed diagonally by a nearly 
straight eye-line, extending on each side from the junction of the cheeks and glabella in front towards 
the lateral angles, cutting the posterior margin a little within the angles, and usually exhibiting a small 
ocular (?) tubercle in the middle. Type of the genus 7’. seticornis (His. Sp.), 7. Bucklandi (Bar.) 
In my Synopsis of the Silurian Fossils of Ireland, I pointed out the course of the eye-line in this 
genus, which separates it at once from Trinucleus, and renders it probable that the small tubercle in the 
middle of the cheeks in the 7’. seticornis, T'. fimbriatus, &c. are true eyes. 
The furrows at the base of the glabella also are distinctive from that genus. Mr Salter, (Mem. 
of the Geol. Survey, Vol. II. Pt. 1. p. 335), speaking of Hawle and Corda’s work, says, ‘“ Tetrapsellium 
is distinguished from Zvinucleus solely by a swelling in the axal furrow of the head; it is almost iden- 
tical else with 7. seticornis:” this might mislead the English reader with the idea that the present genus 
then was identical with Tetrapsellium; the fact is, however, in his strictures on the Bohemian authors 
Mr Salter overlooks the grand character of their genus, namely, its having but four body-rings (‘ Vier 
Leibringe,” H. and C. Monog. p. 42. 8th line). 
TRETASPIS FIMBRIATUS (Murch. Sp.) Pl. 1. E. fig. 16. 
Syn. and Ref—Trinucleus fimbriatus (Murch.) Sil. Syst. t. 23. f. 2. 
Sp. Ch.—Cephalic shield very minutely granulated, more than twice as wide as long; gladella pyriform, 
moderately convex, one-third longer than wide, two short segmental furrows at each side of the base; 
cheeks moderately convex, with the straight diagonal eye-line and central ocular tubercle almost imper- 
ceptible ; margin very narrow, radiatingly furrowed, except the lateral angles, which are directed outwards 
and downwards, and bear irregular punctures, as in the other species; thorax shorter than the glabella ; 
axis two-thirds the width of the pleuripedes; pygidiwm three times as wide as long, with seven axal and 
six lateral segments. Most usual length of shield about two lines, of which the fimbriated margin is about 
half a line; width, exclusive of the spines, five lines. 
Position and Locality —Llandeilo flag, Pen Cerrig, three miles N. of Builth, Radnorshire ; Bryn 
Melyn, near Bala, Merionethshire. 
Explanation of Figures.—P\. 1. E. fig. 16. Entire specimen, natural size, from Pen Cerrig.—Fig 16 a. 
Ditto, magnified three diameters. 
