some British Fossil Crustacea. 4.03 
(Descriptions of new genera and species of Trilobites.) 
Chasmops (M‘Coy), n. g. 
KEtym. yaspa, hiatus, and op, oculus. 
Gen. Char. Cephalic shield subsemicircular, lateral angles pro- 
duced backwards in triangular spines ; 
glabella large, clavate, frontal portion 
very wide, transversely oval, only two 
distinct pairs of lateral segmental lobes, 
the anterior pair very large triangular, 
posterior pair small, middle pair obsolete 
or reduced to a minute tubercle; neck- 
segment strong: cheeks small triangu- Cephalic shield of 
lar: eyes small, rounded, “ hiant,” corre- Chasunoye: 
sponding in height to the middle portion of the first lateral 
lobe of the glabella ; eye-lme encircles the front of the gla- 
bella close to the margin, descends with an inward inclination 
to the eye, extending from behind the eye directly outwards 
to the lateral margin, which it cuts considerably in advance of 
the angles ; thorax of eleven joints (fid. Hichwald) ; pygidium 
obtusely rounded, posterior margin defiected, anterior margin 
wider than the posterior ; axis of about ten ribs, lateral ribs 
about two less, duplex. 
The Calymene Odini of Eichwald may be looked upon as the 
type of this genus. It differs from Calymene in the glabella beng 
so much wider in front than at the base, in the anterior lateral 
lobes being largest, in having but eleven (?) body-segments, and 
in its eye-line cutting the external margin in front of the angles, 
agreeing only in the structure of the eyes; these differences be- 
come agreements when compared with Phacops, from which it 
differs in the structure of the eyes. Of those organs in the pre- 
sent genus and in Calymene nothing is known beyond that they 
were of so tender and delicate a nature as readily to fall out after 
death, and are never found in the fossil state, their position 
being indicated by a hole, roughly filled by the matrix, forming 
the “ hiant”’ eyes of systematists ; in Phacops, on the contrary, 
the cornea is of extraordinary strength, and so firmly united to 
the rest of the cephalic shield, that no matter how much crushed 
the specimens may be, the eye always remains, and from its con- 
stant presence, coarse reticulation and large lenses, gives an ap- 
propriate name to the genus, and one which is in antagonism 
with that I have adopted for the present group: Chasmops differs 
besides from both those genera in the almost complete suppres- 
sion of the middle pair of segmental lobes of the glabella. 
27% 
