PLETHODUS. 107 



Chalk of Kansas. 1 In this fish (Text-fig. 33) the trunk is elongated and laterally 

 compressed, with an extended dorsal fin occupying the greater part of the back, a 

 small remote anal fin, and a forked caudal fin. The vertebras resemble those of 

 Pachyrhizodus (p. 124). The paired fins are small, the pectorals inserted high on 

 the flank, and the pelvic pair far back. The scales are large, elliptical, and 

 smooth. 



Genus PLETHODUS, Dixon. 



Plethodus, F. Dixon, Geol. Sussex, 1850, p. 366. 



Generic Characters. — Skull deep and laterally compressed. Gape of mouth 

 small ; premaxilla apparently fused with the short rostrum, this, the maxilla, and 

 dentary bearing minute teeth ; ectopterygoid very slender, also with minute teeth ; 

 upper and lower dental plates consisting of clustered minute vertical tubules. 



Type Species. — Plethodus expansus from the English Chalk. 



Uemarlcs. — This genus is known chiefly by the detached dental plates ; but 

 imperfect skulls of the smallest species have also been discovered. The dental 

 plates, which were originally referred by Dixon to a Cestraciont shark, usually 

 exhibit their tubular structure and a bony base of open texture ; but the tubular 

 tissue is sometimes changed to a dark translucent substance which shows only 

 small cavities like those of the underlying bone. 



1 . Plethodus expansus, Dixon. Plate XXII, figs. 1 — 5. 



1850. Plethodus expansus, F. Dixon, Geol. Sussex, p. 366, pi. xxxiii, fig. 2. 

 1888. Plethodus expansus, A. S. Woodward, Proc. Geol. Assoc, vol. x, p. 331. 



1899. Plethodus expansus, A. S. Woodward, Ann. Mag. Nat. Hist. [7], vol. iii, p. 354, pi. xiii, figs. 

 1—4. 



1900. Thryptodus sp., F. B. Loomis, Paheontogr., vol. xlvi, p. 235. 



1901. Plethodus expansus, A. S. Woodward, Catal. Foss. Fishes B. M., pt. iv, p. 81. 



Type. — Imperfect lower dental plate from a Turonian zone ; Brighton Museum. 



Specific Characters. — The type species, known only by lower and upper dental 

 plates, which sometimes measure 10 cm. in length. Lower dental plate leaf- 

 shaped, one end being comparatively broad and gently rounded, the other end 

 nearly pointed ; its grinding surface slightly convex except near the pointed end, 



1 See especially O. P. Hay, " On certain Genera and Species of North American Cretaceous 

 Actinopterous Fishes," Bull. Amer. Mus. Nat, Hist., vol. xix, 1903, pp. 26—47, with figs. Also see 

 descriptions and figures of skulls under the names of Thryptodus, Pseudothryptodus, and Syntecjmodus, 

 by F. B. Loomis, Palseontogr., vol. xlvi, 1900, pp. 229—236, with figs. 



