VEETEBRATES. 251 



GENUS GYRACANTHUS, Agassiz. 

 GYRACANTHUS? CORDATUS, St. J. and W. 



PI. XXVI. Fig. 4. 



Spine of small size, very slightly arched along the anterior angle 

 and gradually tapering, transversely triangular or cordate in section, 

 Lateral surfaces broadly convex transversely, meeting in a narrow 

 ridge forming an acute angle at the anterior edge; postero-lateral 

 angles rounded and merging into the broad, moderately excavated 

 posterior face. Exposed portion of the lateral surfaces ornamented 

 with stellate tubercles closely set longitudinally, but showing a 

 more or less distinct arrangement in oblique rows, with a forward 

 and upward course. Pulp cavity relatively large, nearly circular in 

 section, with a thin wall in the axis of the posterior face and thick- 

 ened at the lateral angles, opening below in a deep furrow extending 

 to the proximal extremity. 



The description is based upon a single specimen, which shows 

 about 5.5 centimetres of the length of a small spine. Both extrem- 

 ities have been broken away, and the specimen is otherwise disfig- 

 ured by abrasion, so much so, indeed, as to render the surface 

 features indistinct over the greater part of the lateral surfaces, 

 which appears to be closely covered with minute tubercles; how- 

 ever, these latter on one side reveal the oblique disposition charac- 

 teristic, or at least prevalent, in Gyracanthus, and which, together 

 with the contour of the posterior face, seem to warrant its reference 

 to that genus. In the more abraded surface areas, where 

 the tubercules are worn away and preserving only their bases, the 

 obliquity is even more readily traced in exactly opposite direction 

 to that characteristic of Gyracanthus ; again, they have the appear- 

 ance of irregular disposition, which, in connection with the rather 

 marked keeled condition of the anterior edge, suggested resemblance 

 to Geisacanthus. But the posterior face is quite unlike that of the 

 latter genus, being simply excavated or channeled longitudinally and 

 without the median angle or ridge characteristic of both Gyracan- 

 thus and Geisacanthus. In the distal half of the best preserved 

 side, near the postero-lateral angle, the tubercles are laterally com- 

 pressed and closely impacted at their sides, while considerably 

 spaced "longitudinally, with a nearly transverse disposition ; but 



