PHACOPS. 19 



General form. — A large species ; foreign specimens often attaining a length of 2£ 

 inches, and some of our English fragments indicate a still larger size. 



The head occupies fully one third of the whole length, and is rather more than a 

 semicircle, and very convex. The inflated glabella occupies more than half the width, 

 taking its measure at the wide front, from which the straight sides converge at an angle 

 of 85°. It is about as wide as its whole length, including the neck-lobe. The glabella 

 is very much rounded in front, and scarcely overhangs the narrow linear margin. Its 

 whole surface is covered with large, coarse tubercles, at equal distances, scarcely more 

 than their diameter apart. There are but slight traces of the upper furrows ; but the 

 tumid glabella is strongly separated from and overhangs the linear basal lobe, which has 

 not distinct lateral tubercles, but in the internal cast shows deep pits on either side, 

 above and below the lobe. The neck-segment is strong and broad, wider than the basal 

 lobe. The neck-furrow is continued round the smooth cheek, and separates a broad, 

 strong margin, leaving a subtrigonal space, much of it occupied by the great eye, which 

 varies from half to more than half the length of the cheek, and is placed rather behind the 

 middle of it. The eye, from the depression of the upper eye-lobe, is subhemispheric, not 

 greatly curved, but strongly convex exteriorly, and covered by about fifty-four strong, 

 prominent large lenses, — in rows shortening towards either side ; about five in one of the 

 central vertical rows. Externally the lenses are very convex. 



Foreign specimens show us that the cornea of this species is very thick, and rises into ridges 

 between the lenses ; and it apparently thickens by age internally. At least old specimens (fig. 9) have the 

 substance much thicker, as shown by the great projection of the casts of the supporting cups (fig. 12*), 

 which of course are the spaces occupied by the soft substance which lay underneath the lenses. 



In some instances, probably in younger individuals, the projection of the cups is considerably less 

 (fig. 13). In others again, the cups project so little, and the cornea is so thin, that there is little difference 

 of level between the ridges and the cups (fig. 1 la). I do not think these differences of proportion, nor even the 

 varying numbers of the lenses in different individuals, at all tend to constitute distinct species. Steininger 

 gives us 46 to 50 lenses in one variety, 87 in another, and 130 (probably for the two eyes) in a third. 

 But he has not pointed out any clear distinctions in the species to accompany these differences in the eye. 

 A specimen of C. bufo, Green, which is a closely allied form, and may possibly be only an extreme variety 

 of our species, has 66 lenses in each eye. 



The eye is elevated, the lentiferous surface not sunk in a furrow, but standing 

 prominently out from the cheek, and overhanging its own base (fig. 11). Head-angles 

 rounded. Obscure traces of the facial suture occur below the eye, but practically they 

 are soldered, and the head does not part at the sutures at all. 



Thorax (in German and Spanish specimens) with the pleurae much bent down, so 



* These cups in the cast of a Trilobite's eye occupy the place of the vitreous body, according to Dr. 

 Burmeister's explanation of the eye-structure. ' Organiz. of Trilobites,' Ray Soc. edit., pi. vi, fig. 4 d. 



