some British Fossil Crustacea. , 403 



{Descriptions of new genera and species of Trilobites.) '' ' ^ 

 Chasmops (M'Coy), n. g. 



Etym. ')(ao-fjLa, hiatus, and w^/r, 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- asmops. 



sponding in height to the middle portion of the first lateral 

 lobe of the glabella ; eye-line 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. Eichwald) ; pygidium 

 obtusely rounded, posterior margin deflected, 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 being 

 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* 



