OF THE SPECIES. 85 



proportionalily smaller ; its axis has either no articulation at all, or a distinct one, very 

 rapidly narrowed posteriorly and even at the commencement rather narrower than the last 

 ring of the hody ; the external terminal angle more or less prominent. 



Remark. — The species of this genus are amongst the largest, but also the rarest Trilobites, and 

 seem to be peculiar to the upper or central Silurian strata. I regret to say that I have only been 

 able to examine casts or imperfect specimens. I recognized the granulation, however, iu botli most 

 distinctly, where there were any remains of shell; the specimens deficient in shell, on the other hand, 

 always appear quite smooth. According to the proportion of axis and lateral lobes, they furnish us 

 with two subdirisions, which most authors enumerate as distinct genera. 



A. BipJeura, Green. — The outer end of the facial suture cuts in half the posterior 

 angle itself. The joints of the axis are not broader than the lateral lobes, and very distinctly 

 separated from the latter ; the latter have a process at the lower and outer extremity, with 

 which they passed beneath the open margin of the cephalic shield during the rolling-up 

 process. The posterior margin of each ring of the axis is strongly reflexed, and the ring in 

 itself alone is highly arched. 



Caudal shield slightly pointed or produced forwards, the axis without joints, the sides 

 even and ribless. 



1, H. Decaji : Scuto capitis dilatato, dimidio latitudine vix longiori ; oculis ellipticis ; annulis 

 trunci convexis, iu margine postico reflexo dilatatis. Long. 2K". 



Rpf. — B'qüeum Dekai/i, Green, Moh. 79, Figs. 8, 9. Bronn, Letluea, i, 1 13. 101 , Plate 

 IX, Figs. 6, 7. Harlan, Med. and PJii/s. Research. 304. Emmr. i)m. 42, IV. 

 RIiLNE Edw. Crust, iii, 316. Bronn, Leonh. and Br. Jahrbuch. 1840, pp. 447 et seq. 



Found in different parts of North America ; amongst other places at Lockport, Madison, 

 Steuben, Cazenovia, Rochester, all situated in the state of New York ; also at Northumber- 

 land, in Pennsylvania, and Mount Hope, in the vicinity of Baltimore. I have only had an 

 opportunity of examining the two plaster casts (Nos. 30 and 31) of Green's fragments, and 

 cannot therefore give an accurate description. The distinct granulation and the acute 

 margins of the body rings render it certain that the calcareous shell remained in the actual 

 specimens, which fact agrees very well with the absence of joints at the caudal axis. These 

 joints are probably wanting only at the upper surface, and are visible on the inner surface, 

 as is shown by the smooth individuals without a calcareous shell. Green's statement of 

 there being fourteen rings is based upon an error, as the models have only thirteen ; for that 

 which appears to be the first, is the thickened posterior margin of the cephalic shield. 



B. Trinierus. — The outer extremity of the facial suture meets the margin rather before 

 the angle of the cephalic shield, externally. The joints of the axis are broader than the 

 lateral lobes, very slightly separated from the latter, and not reflexed at the posterior margin; 

 a distinct transverse furrow, which also continues over the anterior surface of the lateral 

 lobes, divides the margin of articulation from the true ring. 



Caudal shield very prominent at the end, the axis distinctly jointed, the sides furnished 

 with ribs. 



