80 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



And sometimes, when a tubercle arises in an intercostal space and does 

 not fuse with its neighbors on either side, its base may become elon- 

 gated, other tubercles succeed behind it, and thus a new longitudinal 

 ridge is formed. Owing to the increase in width of the spine toward 

 the base, it rarely happens that after a ridge is once formed it is crowded 

 out. 



The slight concavity of the anterior margin, as seen in the type, 

 might lead one to suppose that the spine was actually curved forward, 

 but this appearance is in all probability due to distortion. The spine 

 is preserved for a length of 16 cm., and has a maximum width of 

 4.5 cm. Its thickness at the base where the cross-section shown in 

 text-figure No. 8 is taken, was probably about 1.8 cm., — making due 

 allowance for the effects of crushing ; and the pulp cavity remains open 

 for a distance of 12.5 cm. The two parallel costse extending along 

 the anterior margin — it is not sharp enough to be called an edge — 

 differ in no respect from the rest. 



To this species is also referred a smaller and more fragmentary speci- 

 men belonging to the Wachsmuth Collection in the Museiun of Com- 

 parative Zoology, obtained thirty years ago from the Kinderhook beds 

 of Burlington, Iowa. That it is a young example, and not the distal 

 end of a fully grown spine, is apparent from several reasons, such as 

 the position of the pulp cavity as seen in cross-section, the straight 

 course of the longitudinal costae, and general delicacy of the specimen. 

 As many as twelve parallel costa; occupy a space only 7 mm. wide. 

 The nodes are separated by about the same intervals relative to tlie 

 size of the costse, as in the adult spine just described ; their striation, 

 however, is much obscured by weathering or wear. An enlarged view 

 of its ornamentation is shown in text-figure 8 B. 



Formation and Locality. — Kinderhook Limestone ; Iowa. 



Ctenacanthus lucasi, sp. nov. 



Plate 6, Fig. 1. 



The spine which is shown of slightly less than the natural size m 

 Plate 6, Fig. 1, unfortunately lacks the greater part of the exserted 

 portion, but so far as can be judged from the curvature, form of cross- 

 section, and oblique line of insertion, it was of the same general form 

 as C. depressus and C. venustus from tlie same horizon. Hence its 

 position may be interpreted as having been in advance of the first 



