140 IOWA GEOLOGICAL SURVEY 



Spines of moderate size having the distal portion essentially 

 as in the preceding species, but the basal or inserted portion 

 curving forward and extended into a rounded anterior projec- 

 tion or "shoulder", comparable in a way to that observed in 

 Stethacanthus and some species of Physonemus. The longi- 

 tudinal costae are finer and more numerous than in the typical 

 species, and are slightly sinuous in direction toward the base. 



Formation and locality. Hamilton limestone ; Milwaukee, Wis- 

 consin, and corresponding Cedar Valley limestone of Buffalo, 

 Waterloo and Waverly, Iowa. 



Genus acantholepis Newberry. 



Spines in reality compound, consisting of two closely applied 

 segments, a small spatulate anterior and a major, regularly 

 arched posterior portion, the latter representing the usual type 

 of dorsal fin-spine, and the former corresponding to its support- 

 ing cartilage, as seen in modern Holocephali. The major seg- 

 ment, or fin-spine proper, sometimes attains a large size, is of 

 triangular section in that the anterior margin is acute, the lateral 

 faces flattened and slightly divergent from each other toward 

 the base, and the posterior face abruptly truncated or becoming 

 slightly hollowed toward the line of insertion, where the walls of 

 the basal portion suddenly flare outwards on either side, and 

 form a subpyramidal figure. Seen in profile, the exserted por- 

 tion is gently arcuate, tapering distally to an acute point, its 

 sides ornamented with numerous, closely spaced, finely stellate 

 tubercles, which sometimes exhibit an arrangement parallel to 

 successive growth-lines. Tubercles along either side of the pos- 

 terior margin enlarged so as to form a double row of denticles 

 directed at right angles to the margin. 



Inserted portion of major segment slightly curved backward 

 and gradually narrowing, with gently rounded proximal extrem- 

 ity, and line of apposition with anterior spatulate segment nearly 

 rectilinear. The smaller anterior segment has the distal portion, 

 which is partially exserted, gradually tapering, and the inserted 

 proximal portion evenly rounded; its ornamentation identical 

 with that of the major portion, or fin-spine proper. Lateral walls 

 of both segments very thin, and hence usually found closely 



