Crusracea. | LOWER PALAZOZOIC ARTICULATA. 169 
Genus. ASAPHUS (in a wider sense than Brong.) 
Gen. Char.—Head and tail nearly equal; eyes with a firmly fixed, thick cornea, with a smooth external 
surface ; facial suture cutting the posterior margin of the cephalic shield within the lateral angles ; thorax with 
eight to ten segments, haying large facets and distinct, wide, nearly straight pleural grooves, not reaching the 
margin ; glubella indistinctly defined in front ; pygidiwm with the segments usually indistinctly marked, the axis 
generally distinct and annulated, when traceable, always elongate conic (of the ordinary type). 
Contains among others the following subgenera, Ist, Asaphus (not British, taking the A. cornigerus as the 
type) ; 2nd, Lsotelus. 
Subgenus. ISOTELUS (Dekay). 
Gen. Char.—Large, elliptical ; buckler semielliptical, with the angles rounded, or produced backwards into 
spines; glabella indistinctly defined ; eye-/ines meeting at an acute angle at ‘the front margin, thence diverging 
backwards, slightly approximating again about the middle of their length, where the “hiant ” eyes are situated, 
and again diverging to cut the posterior margin near the angles; thorax of eight segments, axis as wide or 
wider than the lateral lobe, the ends of the pleurze rounded, with a strongly marked triangular facet and pleural 
groove; pygidium resembling the buckler in size and shape, trilobed, and generally with a broad smooth mar- 
gin; axis and lateral lobes with fine segmental furrows, or smooth. 
There seem to be two types, Ist, the Isorenus (gigas, &c.), having the angles at the head rounded, and 
no distinct segmental divisions of the pygidium ; 2nd, (Asaphus tyrannus, &e.) having the lateral cephalic angles 
produced into spines, and both the axis and lateral lobes of the pygidium distinctly defined and divided into simple 
segments. After acareful review of all the species I do not, however, feel competent to separate them ; if it should 
be found convenient to distinguish them as subgenera, the name Jsofe/us should be retained for the former, and 
the latter might be called Basilicus, as suggested by Mr Salter, in the 2nd Decade of the Geol. Survey. Basi- 
licus differs from Ogygia, which it closely resembles in shape, in the facets of the pleurze, as well as the simple 
(not duplex) furrows of the pygidium ; the glabella is more prominent than in Jsotelus proper. 
IsoTELUS AFFINIS ({M‘Coy). Pl. 1. F. fig. 3. 
Syn. and Ref—Tsot. gigas, I. planus and I. Powisii of Portk. Geol. Rep. (not of the writers he refers to) 
Pl. 6. f. 1. and t. 7. f. 2 and 3. 
Sp. Ch—Axis of the body only slightly exceeding the pleurz in width; plewre gently arched downwards 
at a very obtuse angle; from about half way between the axis and the extremity, a large pleural furrow reaches 
from the axis to about one-third of the truncated extremity of each ; pygidium flattened, semielliptical, or slightly 
trigonal, from the straightness of the sides; avis narrow, sharply defined, gently convex, reaching as far as the 
concaye space round the margin. 
In general proportion this resembles the Jsotelus gigas (Dekay), from all the varieties of which it is dis- 
tinguished when specimens of the same size are compared, by the much greater flatness or depression of all its 
parts, the long, narrow, sharply defined axal lobe of the pygidium, and the much greater length of the pleural 
groove of the pleurze (nearly double that of the J. gigas), and distance of knee from the axis, as well as their slight 
degree of deflection (being bent at nearly right angles at one-third from the axis in J. gigas). The pygidium 
differs from that of the 7. Powizsii (Murch. Sp.) by the absence of all segmental furrows on the lateral lobes, 
except the first, and by the more pointed outline and narrow margin. 
Position and Locality—Not uncommon in a schist over the iron-works at Tremadoc, Merionethshire ; 
very similar in appearance to that at Pomeroy, county Tyrone, which afforded the species to Col. Portlock. 
Explanation of Figures.—P1. 1. F. fig. 3.' Pygidium and thorax natural size, from the fine slates over 
the iron-works of Tremadoe. 
