08 FOSSIL FISHES OF THE ENGLISH CHALK. 



Genus LEPTOTRACHELTJS, W. von der Marck. 



Leptotrachelus, W. von der Marck, Palaeontogr., vol. xi, 1863, p. 59. 

 Trisenaspis, E. D. Cope, Bull. U.S. Geol. Surv. Territ., vol. iv, 1878, p. 07. 



Generic Characters. — Vertebrae between GO and 80 in number. Dorsal fin 

 occupying not move than the middle third of the back; pelvic fins opposed to this, 

 bul anal fin more remote; caudal fin forked. Most of the dermal scutes triradiate, 

 some narrow heart-shaped, and adjoining longitudinal series scarcely if at all in 

 contact ; a paired series along the dorsal and ventral borders, and another along 

 the lateral line; no intermediate squamation. 



Type Species. — Leptotrachelus armatus (W. von der Marck, Palaeontogr., vol. xi, 

 1863, p. 50, pi. x, fig. 3), from the Upper Cretaceous of Sendenhorst, West- 

 phalia. 



Remarks. — The finest known examples of species of this genus have been 

 obtained from the Upper Cretaceous of Sahel Alma, near Bey rout, Syria. Some of 

 them are described in detail in the ' Catal. Foss. Fishes B. M.,' pt. iv, pp. 174 — 

 1 84, and form the basis of the accompanying restored sketch of L. triqueter (Text- 

 fig. 15). One specimen in the British Museum (Text-fig. 16) encloses a compara- 

 tively large undigested fish, which maybe regarded as evidence of a distensible 

 stomach. The remains of the only known species from the English Chalk are 

 important as displaying the osteology of the head. 



1. Leptotrachelus elongatus (Agassi/). Plate XVI. 



1835-44. Dercetis elongahis, L. Agassiz, Poiss. Foss., Feuill., p. 55, and vol. ii, pt. ii, p. 258, pi. lxvi a, 



figs. 1, 2, 5 — 8 (non figs. 3, 4). 

 1879. Dercetis elongatus, W. Davies, Geol. Mag. [2], vol. vi, p. 145. 



1888. Derceti* elongatus, A. S. Wood ward, Proc. Geol. Assoc., vol. x, p. 318, pi. i, fig. 7. 

 1901. Leptotrachelus elongatus, A. S. Woodward, Catal. Foss. Fislies B. M., pt. iv, p. 184, pi. xii, 



fig. 4. 



Type, hnperfect head and trunk, probably from one of the Turonian zones; 

 British Museum. 



Specific Characters. -A slender species, probably attaining a length of about 

 60 cm., hut usually smaller. The postorbita] region of the skull about as broad as 

 Ion--, this occupying the hinder quarter and the orbit the next quarter of the 

 crania] length ; the length of the head with opercular apparatus contained between 

 five and six times in the total length of the fish to the base of the caudal fin. 



