12 Prof. E. J. Chapman on new Trilobites from Canadian Rocks. 



(Is.) affinis, M'Coy (including Is.gigas, Is. planus, and Is. Powisii 

 of Portlock) — in having, with other opposing characters, the 

 genal angles of the head-shield extended into horns. 



(2.) It differs from A. tyrannus, Murchison, A. Powisii, 

 Murchison, and A. ingens, Barrande — in having, with other 

 opposing characters, the branches of the facial suture united 

 above the glabella on the upper surface of the head- shield. 



(3.) It differs from A. nobilis, Barrande, in wanting the 

 curved furrows on the axis of the pygidium, as exhibited by that 

 species ; and also by the greater number of the segment-mark- 

 ings on the side-lobes of its pygidium, as well as by the general 

 outline of the facial suture, and other characters. 



(4.) It differs from A. extenuatus, Waldheim, by the obtuse 

 outline of its cephalic shield, and by other marked characters. 



(5.) It differs from A. (Is.) laticostatus, Green, — the genal 

 angles of which are unknown, — by its thorax being nearly of the 

 same length as its head-shield, and by the greater number of 

 segment-markings on the side-lobes of its pygidium, as well as 

 by other characters. 



(6.) It differs from A. ovatus, Portlock, by the presence of 

 segment-markings on the side-lobes of its pygidium. I am not 

 acquainted with the head-shield of A. ovatus, and I cannot here 

 obtain a copy of Colonel Portlock' s Report in which the species 

 is figured. 



(7.) It differs from A. angustifrons, Dalman, and A. frontalis, 

 Dalm., by the greater development of its genal points; Dal- 

 man's species being placed by him under his subdivision " Mu- 

 tici," comprising the forms with rounded or but slightly pointed 

 genal angles. I am not sufficiently acquainted, however, with 

 these Swedish species to name any other distinguishing charac- 

 ters ; and I have no means of procuring here a copy of Dalrnan's 

 ' Palseaden/ in which the species are described. 



(8.) It differs from A. Iowensis, Dale Owen, by its genal 

 points reaching only to the middle instead of to the end of the 

 thorax ; by its facial suture being pointed, instead of curved, 

 above the glabella ; and by the presence of segment-markings 

 on its pygidium. 



The head-shields of A. grandis, Sars, A. Fournetti, De Ver- 

 neuil, and A. latimarginatus, Hall, are still unknown. 



(9.) It differs from A. Cianus, Barr. (the genal angles of which 

 are unknown), by its slightly raised glabella and by its narrow 

 body-axis. 



(10.) It differs from the imperfectly-known A. glabratus and 

 A. contractus of Barrande, by its feebly raised glabella, by the 

 greater number of segment-markings on the side-lobes of its 

 pygidium, and probably also by the direction of its facial suture. 



