VERTEBRATES. 423 



by once or twice their diameter ; in the front ridges they are apparently 

 smooth, possibly the result of wear, and in other parts both the trans- 

 verse and round forms are associated on the same rib. It is exceedingly 

 difficult to portray the minutiae of details of the tuberculose ornamenta- 

 tion here observed, except through the medium of illustrations, of which 

 we have made such use as seems to meet the requirements of their iden- 

 tification. The base appears massive in front, of moderate depth, rap- 

 idly converging to the inferior extremity, deeply channeled posteriorly 

 by the pulp cavity which occupies more than half the diameter of the 

 middle portion of the spine, where it presents an elliptical transverse 

 section. The posterior face is occupied by a strong median ridge slightly 

 channeled on either side, and which becomes exceedingly promiueut on 

 reaching the middle of the spine ; the posterolateral angles are sharply 

 defined for a short distauce below the apex, where they are armed with 

 a row of prominent, obtusely hooked denticles, slightly compressed lat - 

 erally and closely approximated, or separated by spaces less than their 

 greatest diameter; lower, the posterior angles appear rounded and des- 

 titute of denticles. 



The solitary example which we have examined of the present form, 

 represents a spine probably seven inches in length, and, though broken 

 and parts missing, sufficient remains, together with the perfect condi- 

 tion of the superficial characters, to show its distinctive characters as 

 contrasted with other forms. One feature not alluded to in the above 

 description and which may be merely individual in its appearance, is 

 the regular bifurcation of the costre along lines of growth corresponding 

 to the line of insertion, and which is partially shown in the illustration, 

 PI. XIV, fig. 2 A. This feature is apparent in both faces, and in regard 

 to the relative obliquity of these lines of growth and the present line of 

 insertion, the observations on the growth of spines made in connection 

 with 0. speciosus, are equally applicable to the present form. In the 

 specimen before us the anterior margin for two-thirds or more of its extent 

 from the apex is quite smooth, apparently from wear, and for a short 

 distance from the apex the lateral cosbe are equally smoothed from the 

 same cause, the surface of the ridges, denuded of their tubercles, being 

 delicately striated longitudinally. 



Compared with 0. speciosus, with which it has in common the same 

 general outline and similar style of tuberculose ornamentation, its dis- 

 tinguishing peculiarities consist in its more robust proportions and 

 greater lateral thickness, the double row of tubercles along the anterior 

 ridges and the more prominent denticles arming the postero-lateial 

 angles. 



Position and locality: The specimen described was discovered by 

 Mr. Giles in the upper fish-bed of the Kinderhook formation ; Flint 

 river, near Burlington, Iowa. 



