■1"|<1[,(1BITES OF IHE CUAZV LIMESTONE. 349 



which are produced in our plates. A reexamination of the specimens 

 shows nothing which escaped the acute eye of Mr. Billings, therefore 

 his original description is given /// fo/o. 



" I I Urn us Bayfieldi (n. s. ). 

 " Descrii'tiox. 



"Oblong, distinctly trilobed ; length two or three inches, width 

 three fourths the length, central lobe rather more than one third the 

 whole width. Proportional length of head, thorax and pygidium 

 about as 9, 8, 6)4. The head is transversely oval in outline, the 

 width twice the length, rather evenly convex, most prominent in the 

 center, the front margin broadly rounded, the posterior margin tri- 

 lobed by the dorsal furrows which are extended forward to about the 

 middle of the head, but are distinct for only about one fourth that 

 distance ; they at first incline towards each other and then becoming 

 very obscure, curve outwards ; on each side of the central lobe the 

 posterior margin is nearly straight, as far as the eye, when it gradually 

 curves forward and outward for one half the length of the head, then 

 turning a broad rounded angle, it merges into the front margin. 



" In consequence of the peculiar form of the posterior margin, the 

 genal angle, in this species, is in the front half of the lateral margin 

 of the head. The eye is of moderate size, lunate and within one 

 fourth its length from the margin. The facial suture curves forward 

 so as to cut the front margin considerably within a longitudinal line 

 drawn through the eye : behind the eye its course is remarkable, as it 

 turns outward and runs parallel with the margin, which it reaches at 

 about three fourths the length of the pleura. The lower angle of the 

 eye is distant from the dorsal furrow a little less than half the width 

 of the central lobe of the jjosterior margin. The width of the cheek 

 piece on a line drawn transversely across the head at one third the 

 distance from the eye to the front is nearly equal to half the distance 

 between the eyes. In a vertical view of the head, neither the full 

 width of the cheek piece nor the position of the genal angle can be 

 seen, as the outline is fore-shortened, consecjuently in the figure above 

 given, the width appears less than it really is. 



" Thorax of ten articulations, axis of thorax square, the length and 

 breadth being the same, moderately convex, well defined ; the fulcra 

 of the pleura are at about one third the width of the axis from the dor- 

 sal furrow, the intervening space flat. 



