INVERT EBRATES. 509 



side on a horizontal plane, its breadth being' to its length very neaily 

 as fifty to thirty ; moderately convex, the hight being rather distinctly 

 less than half the breadth, and the most prominent part a little behind 

 the middle ; while the curve over the middle, from its posterior to its 

 anterior margin, forms about a quarter of a circle. Anterior margin, 

 as seen from above, presenting a nearly transversely semielliptic curve, 

 and a subquadrangular outline, as seen in a side view; lateral margins 

 rather narrowly, and regularly rounded in outline, into the posterior 

 side. Axial furrows distinct, converging forward to a point nearly 

 opposite the middle of each eye, where they terminate in little flattened 

 oval depressions. Eyes large, forming nearly semicircular curves, with 

 their posterior ends as near the posterior as to the lateral margins of 

 the head ; each with a broad, very deep furrow around beneath its outer 

 side, so as to form a kind of obtuse shoulder below, from which the 

 cheeks drop off nearly vertically, with a slight convexity of outline, to 

 the inferior margins ; palpebral lobes less elevated than the middle of 

 the glabella, and sloping a little outward, with an even convexity over 

 their whole surface; visual surface forming a rather narrow convex 

 band, and showing (in internal casts) under a magnifier, numerous very 

 minute reticulations. Facial suture cutting the anterior margin dis-. 

 tinctly within a line drawn antero-posteriorly through the 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 of the head, without lines, furrows or other markings, but rather 

 distinct transverse furrows are seen on the rostral shield. Body and 

 other parts unknown. 



Length of head, about 1.20 inch ; breadth of head, 2.47 inches ; hight 

 or convexity, 1.05 inch. Length of eyes, 0.55 inch ; hight of visual sur- 

 face, 0.10 inch; distance between the eyes at posterior and anterior 

 ends, 1.65 inch. 



This species is perhaps most nearly related to the common and widely 

 distributed I. Barriensis of Murchison. It may be readily distinguished, 

 however, 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 pro- 

 portion to its length, and has its lateral margins, as seen from above, 

 much more narrowly and regularly rounded, so that the cheeks do not 

 project any farther out from the eyes posteriorly, than laterally, the out- 

 line 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 narrow in its anteroposterior 

 diameter, and distinctly obtuse, instead of pointed at the lateral extremi- 



