28 CRETACEOUS AND TERTIARY FISH. 



Genus ACRODUS Buckland. 



Acrodus Buckland, Geol. Mineral, Ed. 2, II, 1837, p. 47. Type Acrodus 



nobilis Buckland, monotypic. 

 Thectodus Meyer and Plieninger, Beitr. Pal. Wurttemb., 1844, p. 116. Type 



Thectodus crenatus Meyer and Plieninger, first species. 



Teeth conic, non-cuspidate, crown mostly striated, with one 

 principal elevation, and one or more lateral prominences in either 

 side diminishing outwards. Root much or moderately depressed. 

 Siymphyseal teeth few, relatively large. Notochord persistent. 

 Two large hooked-shaped semi-barbed dermal spines immediately 

 after each eye. Shagreen sparse, consisting of small, conical, ra- 

 diately-grooved tubercles, sometimes fused into groups of three. 

 Dorsal fin-spines longitudinally ridged and grooved, ridges not 

 denticulated, and two posterior longitudinal series of denticles, not 

 marginal, but placed together mesially. Anterior dorsal spine 

 longer and more slender than posterior. 



This genus is closely related to Hybodus Agassiz, and differs 

 only in the rounded and non-cuspidate character of its teeth. The 

 dorsal fin-spines are also scarcely to be distinguished from those 

 assigned to the same, their determination being based on their 

 association and stratigraphical order. Altogether, possibly 55 

 species may be referred to this genus. 



Acrodus humilis Leidy. 



Acrodus humilis Leidy, Proc. Acad. Nat. Sci. Phila., 1872, p. 163. Lime- 

 stone from New Jersey cretaceous. 

 Leidy, Rep. Geol. Surv. Terr. Hayden, 1873, pp. 300, 352, PI. 37, fig. 5 

 (type). 



Tooth depressed. Crown smooth, evenly convex over its entire 

 upper surface, greatest width not quite one-third its length, ends 

 rather angular, and long edges broadly projecting over narrow 

 base, with under surfaces nearly level or but slightly inclined. 

 Upper surface of crown, though with smooth aspect, shows retic- 

 ulations, which, though rather coarse along median axis soon 

 become smaller, and along edges are very minute. Base width 

 about one-half that of crown, with ridge anteriorly and groove 



