HISTIONOTUS. 77 



Type Species. — Histionotus angularis, from the English Purbeck Beds. 



Remarks. — Histionotus is noteworthy for the great development of the sliinr- 

 canals on the head and preopercnhmi. It is known only by the type species and 

 by two or three others from the Lithographic Stone (Lower Kimmeridgian) of 

 Bavaria and France. 



1 . Histionotus angularis, Bgerton. Plate XVII, figs. 1 — 5. 



1854-55. Histionotus angularis, P. M. G. Egertou, Aim. Mag. Nat. Hist. [2], vol. xiii, p. 434, and 



Figs, and Descript. Brit. Organic Bemaius (Mem. Geol. Surv.), dec. viii, no. 5, pi. v. 

 1889. Histionotus angularis, J. C. Mansel-Pleydell, Geol. Mag. [3], vol. vi, p. 241, pi. vii. 

 1895. Histioiiottix angularis, A. S. Woodward, Catal. Foss. Fishes B.M., pt. iii, p. 174. 



Ti/pc. — Fish, wanting tail; British Museum. 



Specific Characters. — Attaining a length of about 20 cm. Length of head with 

 opercular apparatus slightly exceeding its maximum depth, and occupying about 

 one-quarter of the total length of the fish ; length of trunk equalling twice its 

 maximum depth, and the dorsal angulation measuring approximately 148°. Head 

 and opercular bones and large postclavicular plates externally ornamented with 

 fine, closely arranged rugae of enamel. Fin-rays stout and smooth ; pectoral fins 

 scarcely twice as large as the pelvic pair, and the latter arising in advance of the 

 middle point of the trunk ; dorsal fin consisting of at least 25 rays. Pectinations 

 of the scales delicate and confined to their hinder margin, but conspicuous in all 

 regions of the trunk. 



Description of Specimens. — The. type specimen is so imperfectly shown, with 

 the caudal region so erroneously restored, in Egerton's original figure that it is 

 re-drawn in PI. XVII, fig. 1. The head-bones are much crushed and broken, but 

 several can be recognised ; the ventral region of the trunk is slightly deepened by 

 crushing; and the caudal region is only vaguely indicated in impression, with a 

 few displaced remains of the fulcra of the anal fin. The characters of the species 

 are better seen in a fine though distorted specimen (PI. XVII, fig. 2) described by 

 Mansel-Pleydell (loc. cit., 1889), and various features are shown by several more 

 imperfect specimens in the British Museum and the Dorset County Museum. 



The deep laterally compressed skull is remarkable for the great development 

 of the slender ethmoidal region, which is as long as the frontals. The parietals 

 (PI. XVII, fig. 3, pa.) are conspicuous behind, each slightly longer than broad, and 

 covered with the rugose enamelled ornament, which tends to an antero-posterior 

 direction in the anterior half but is disturbed by a large transverse slime-canal in 

 the posterior half. The squamosals have not been clearly observed. The frontals 

 (fig. 3,/r.) are about twice as long as the parietals, slightly arched over the large 

 orbit and not much tapering in front. In their postorbital expansion the rugose 

 ornament is sparse, but in their interorbital portion it is more conspicuous and 



