VEBTEBRATES. 443 



appear displaced, as shown in the figure, PI. XVI, fig. 14 a ; the upper 

 half, however, apparently shows nearly a perfect transverse section, 

 as determined by breaking the specimen in several places, by which 

 means we have been enabled to make out the following general char- 

 acteristics : 



Fin-spine recumbent if not imbedded along its entire inferior extent, 

 laterally compressed, subovate in transverse section ; basal or imbedded 

 portion of greater or less depth, anteriorly produced beyond the limit 

 of the anteroinferior shoulder of the exposed body, thick, and coarsely 

 osseous in structure ; exposed body of the spine constricted along the 

 line of union with the base, lateral faces converging to the more or less 

 obtusely rounded anterior margin, and longitudinally costate. Internal 

 cavity ? 



AUACLITACANTHTJS SEMICOSTATUS, St. J. and W. 

 PI. XVI, Pig. 14. 



The base of the spine nearly equals the greatest transverse diameter 

 of the exposed portion, nearly as thick as it is deep and broadly rounded 

 along the lower edge, the striated appearance of the surface being pro- 

 duced by the coarse, fibrous structure, the striae diverging interiorly from 

 the basal or insertion line; the crown or exposed body gradually rises 

 from the anteroinferior point into a long, elevated, obtusely rounded 

 ridge from which the sides gradually diverge, probably gently convex, to 

 the sharply constricted or iubeveled margin along the line of insertion, 

 presenting in connection with the base a subelliptic or long-ovate trans- 

 verse section ; the lateral faces are covered with numerous bifurcating 

 costas, which are larger and more or less irregularly disposed in the 

 middle and anterior portion, gradually diminishing in size posteriorly 

 and apparently more regular and parallel with the basal line, generally 

 smooth, but with occasional traces of minute pectination ; intercostal 

 spaces moderately deep, of variable width, probably averaging half the 

 diameter of the costae. 



The specimen described exhibits about two thirds of the entire length, 

 indicating a spine of at least six inches in length. The peculiar and 

 anomalous feature is the great extent of the inserted base, which prob- 

 ably extended nearly if not quite to the tip, and the semi-costation. 

 The costte are very irregular in the middle and front portion of the 

 spine, often extremely roughened, but in the imperfect state of preser- 

 vation it is difficult to assign the cause of this appearance, though it is 

 apparently indicative of tuberculation ; indeed the obscure traces of 

 pectination would seem to corroborate this last inference. There is no 



