NATURAL SCIENCES OF PHILADELPHIA. 55 



a magnifier numerous very minute reticulations. Facial sutures catting the 

 anterior margin distinctly within a line drawn antero-posteriorly through tin- 

 inner ends of each eye, and intersecting the posterior margin nearly on a line 

 with the middle of each eye. Rostral shield flat, with a subfusiform outline 

 and obtuse lateral extremities; just three times as wide as its antero-posterior 

 diameter. Surface of the internal cast of the whole upper part ol the head 

 without lines or furrows, but rather distinct transverse furrows are seen on 

 the rostral shield. (Body and other parts unknown.) 



Length of head, about 1 20 inches; breadth of do., 2-47 inches; height or 

 convexity, 1-05 inch. Length of eyes, 055 inch; height of visual surface of 

 same, 0-10 inch ; distance between the eyes at posterior and anterior ends, 165 

 inch. 



This fine species is perhaps most nearly allied to the common and widely 

 distributed /. Earriensis of Murchison. It may be readily distinguished, how- 

 ever, by several important differences in the head, which is the only part yet 

 known to us. In the first place, its head is much wider in proportion to its 

 length, and has its lateral margins, as seen from above, much more narrowly 

 and regularly rounded in outline, so that the cheeks do not project any farther 

 out from the eyes posteriorly than laterally, the outline of the lateral margins 

 having almost exactly the same curve as the eyes themselves. Its rostral 

 shield also has a very different form from that of Murchison's species, being 

 narrower in its antero-posterior diameter, and distinctly obtuse, instead of 

 pointed, at the lateral extremities. Our species likewise shows no traces of 

 the furrows on the cast of the upper side of the head, so strongly marked in /. 

 Barriensis. 



It is still more widely removed from /. inxignis of Hall ; and we know of do 

 described species having the head so nearly elliptic in outline (transversely) 

 as seen from above, excepting possibly /. Salteri of Barrande, which, however, 

 differs widely in other characters, belonging, as it does, to the small-eyed 

 section of the genus. 



Locality and position. Grafton, Illinois, from the Niagara division of the 

 Upper Silurian. 



DlTHYROCARIS CARBONARIU S, M. and W. 



We only know this fossil from a specimen showing the caudal appendage?, 

 that is, the telson and stylets.* These are lanceolate in general outline, and 

 rather flattened. The telson seems to be a little shorter than the stylets, and 

 more rapidly tapering toward the extremity. Below it is flat, and has a faint, 

 undefined, obtuse longitudinal ridge along the middle, with on each side an 

 equally undefined, shallow sulcus between this and the lateral margins, which 

 are sharp. On its upper side there is a well defined mesial carina, with a 

 slio-htly concave slope on each side to the lateral margins, thus presenting 

 mueh the form of a broad bayonet. The stylets have each, on the flattened 

 under side, about six or seven small longitudinal ridges, and on the upper side 

 a distinct longitudinal mesial carina, between which and the lateral margins 

 there is on each side a smooth rounded concavity or broad furrow; along eacb 

 lateral margin there are two closely approximated carinas, oae above, and one 

 below with a narrow rounded sulcus between. 



Len-th of telson, about 0-75 inch; breadth of do., 012 inch ; length of sty- 

 lets, about 0-80 inch ; breadth of same near the articulating end .0-1 2 inch 



This species will be readily distinguished from D. Scoulen of McCoy, bj its 

 proportionally broader and more lanceolate stylets and telson, the latter of 

 which is also smooth instead of being marked by oblique divancat.ng stria, - 



and not the whole of that segment. 



1870.] 



