VERTEBRATES. 437 



Asteeopttchitjs St. Ltjdovici, St. J. and "W. 



PI. XVI, Tig. 3,4. 



Spines of medium size, moderately strong, slightly curved backward, 

 very gradually tapering to the distal extremity, near which the opposite 

 margins more rapidly converge, transverse section sublenticular, form- 

 ing an acute angle in front, lateral faces gently arched and slightly 

 converging to the abruptly truncated posterior margin. The anterior 

 marginal carina rounded, prominent, and similar to the longitudinal 

 carina; which occupy the body of the spiue, from which it is separated 

 for the greater length of the spine by a plane space of variable width, 

 but towards the extremity the costse are more crowded and uniformly 

 spaced by about their own diameter; in the middle and lower part the 

 intercostal spaces are of variable width, generally narrower and more 

 uniform in the posterior half, and often equal to more than twice the 

 diameter of the costfe in the anterior half, and delicately striated longi- 

 tudinally ; the broad space on either side of the anterior margin is gen- 

 erally occupied by irregularly disposed minute tubercles, which also 

 occur sparsely in the second and third furrows from the margin, but 

 these do not appear invariably, being absent in some specimens. The 

 lateral cost* rarely bifurcate descending, generally near the apex, 

 enlarging below. Posterior margin traversed by a well-defined median 

 ridge throughout its greater length, but less prominent above, gently 

 depressed on either side and finely striated longitudinally. Postero-lat- 

 eral angles marked by more or less closely arranged and hooked, 

 laterally compressed denticles, those near the distal extremity being 

 perceptibly stronger, more closely approximated aud more strongly 

 hooked downward, while the central and lower denticles are more erect 

 with their apices directed upward; the transition in the reversed direc- 

 tion of the apices of the denticles appears quite suddenly, the inter- 

 mediate denticle of the upper series being more obtusely pointed, the 

 same as is the case with that of the lower series, where they are per- 

 haps generally less uniform in shape. Pulp cavity oval in transverse 

 section, occupying half or more the diameter of the spine in the central 

 portion, but relatively greatly reduced in the upper part, restricted to 

 the posterior half. Base unknown. 



The collections afford several examples of this elegant ich thy odorulite, 

 but they are generally quite fragmentary, the individuals figured being 

 the most perfect we have met with. From the examination of this 

 material, it would appear that these spines present considerable 

 individual variability, such as the variable spacing of the longitudinal 

 costce as also the denticles along the postero-lateral angles, and the 



