36 FOSSIL FISHES OF THE ENGLISH CHALK. 



Gr.uiis ACROGNATHUS, Agassiz. 



Acrognathus, L. Agassiz, Poiss. Foss., vol. v, pt. ii, 1844, p. 108. 



Generic Characters. — Head large, with remarkably large orbits and narrow 

 interorbital region ; mandibular suspensorium nearly vertical ; maxilla expanded 

 behind ; mandible delicate and dentary pointed at symphysis ; teeth minute and 

 clustered. Vertebras between 30 and 40 in number. Pectoral fins delicate and 

 small, inserted above ventral border ; pelvic fins larger and stouter, with 7 or 8 

 rays; dorsal fin in the anterior half of the back, with 10 to 12 rays, none exces- 

 sively elongated ; anal fin small and remote ; caudal fin forked. Scales large, thick, 

 and uniform, not serrated on the hinder border, not deepened or thickened along 

 the course of the lateral line, which is conspicuous. 



Type Species. — Acrognathus hoops, from the English Chalk. 



Bemarks. — The type species of this genus being still very imperfectly known, 

 some characters mentioned in the foregoing diagnosis are derived from observations 

 on a supposed allied species, Acrognathus libanicus. from the Upper Cretaceous 

 of Mount Lebanon (Catal. Foss. Fishes B. M., pt. iv, 1901, p. 244). 



1. Acrognathus boops, Agassiz. Plate X, fig. 3. 



1844. Acrognathus hoops, L. Agassiz, Poiss. Foss., vol. v, pt. ii, p. 108, pi. Ix a, figs. 1 — 4. 

 1888. Acrognathus hoops, A. S. Woodward, Proc. Geol. Assoc, vol. x, p. 323. 

 1901. Acrognathus hoops, A. S. Woodward, Catal. Foss. Fishes B. M., pt. iv, p. 243. 



Ti/p)e. — Imperfect distorted fish, probably from one of the Turonian zones of the 

 Chalk ; British Museum. 



Specific Characters. — The type species, attaining a length of about 12 cm. 

 Length of head with opercular apparatus exceeding maximum depth, probably 

 equalling about half length of trunk from pectoral arch to base of caudal fin. 

 Dentary and angular bones of mandible marked with sharp radiating grooves. 

 Scales quite smooth. 



Description of Specimen. — Apart from an unimportant fragment, also in the 

 British Museum, the type specimen is still the only known example of this species. 

 It is an imperfectly preserved fish shortened by oblique distortion, its upper border 

 being thrust forwards and a little downwards. It is shown of the natural size from 

 the upper and right lateral aspects in PI. X, figs. 3, 3 a. The fish must have been 

 originally round-bodied and rather elongated ; while the size of the orbit is probably 



