76 BRITISH CAMBRIAN TRILOBITES. 



of the tail of Sph. majasculus, 1 but it is, I believe, a tail of Sph. alatus with the 

 preceding thoracic segment attached. In many specimens of r>ph. alatus (see, for 

 example, Plate VIII, fig. 5) the lateral lobes of the tail appear to be almost obsolete, 

 but in such specimens the ends of the thoracic pleurae are not visible and probably 

 the lateral lobes of the tail are also incomplete. In external casts the axis appears 

 to be relatively wider and the lateral lobes narrower than in Plate VIII, fig. G, thus 

 approaching more closely to the proportions shown in the figure of the tail of Sph. 

 alatus given by Linnarsson. 



The only form with which this species is at all likely to be confounded is 

 Sphaerophthalmits major. The latter, however, is generally larger, and is 

 characterised by a relatively narrower axis and wider side-lobes. In the head the 

 fixed cheeks are nearly as wide as the glabella, while in S. alatus they are much 

 narrower. In the tail of S. major the lateral lobes form about three-fifths of the 

 total width ; in S. alatus about one-half; in the former also both the axis and the 

 tail itself are more rounded. The difference in the thorax is not so marked, but 

 even here the axis in S. majusculus seems to form a smaller proportion of the total 

 width ; the fulcrum, moreover, is rather further from the axial furrows. 



Sphasrophthalmus alatus is commonly found with various species of Ctenopyge, 

 but from all of these it is readily separated. Its general form is more convex. 

 The cranidium is at once distinguished by the backward position of the eye, and 

 the fact that behind the eye the fixed cheek is very little expanded. The free 

 cheek can be recognised by its rounded outer margin, the shortness and slenderness 

 of the cheek-spine, and the round eye set close to the posterior margin. The 

 thorax is characterised by its convexity and the breadth of the axis ; and the tail 

 with its wide axis and extremely narrow lateral lobes is quite unlike any other. 



The form described by Matthew as Sphserophthalmus alatus var. canadensis does 

 not appear to differ in any essential features. It has an anterior pair of glabellar 

 furrows, but these are short and seem to be lightly impressed, and their apparent 

 presence or absence may well depend on the state of preservation. There is also an 

 oblique ocular ridge ; this is sometimes seen in our English specimens, more 

 especially in external casts. Matthew describes a second variety, which he calls 

 the "narrow form," in which no anterior glabellar furrow and no ocular ridge are 

 visible, but he ascribes to it a free cheek with the eye set rather more forward 

 than in our specimens and with a long and strong genal spine. He himself 

 remarks on the resemblance of this free cheek to one of those referred by 

 Linnarsson to Ctenopyge, but concludes from its associations that it really belongs 

 to this species. So far as our English specimens are concerned the evidence is 

 clear that Sph&rophthahnus had short cheek-spines and Ctenopyge long ones. 



Synonymy. —According to the strict rules of priority the specific name alatus 

 has no right to stand, for Boeck's description is totally inadequate and he supplies 

 no figure, whereas the figure and description given by Phillips are quite unmistak- 

 1 Geol. Foren. Stockh. Fork, vol. v (1880), pi. v, fig. 11. 



