442 FISHES CHAP. 
The widely-distributed genus Acanthodes (Fig. 251) is repre- 
sented in the Lower Old Red of Scotland, the Devonian of Siberia 
and Canada, the Carboniferous of England and Scotland, and the 
Lower Permian of France, Germany, and Bohemia. Acanthodopsis 
(Coal Measures), and Mesacanthus and Cheiracanthus (Lower Old 
Red) are the remaining genera. 
Order IV. Plagiostomi. 
Head prolonged in front of the ventrally-situated mouth as a 
more or less prominent preoral rostrum, vertebral column consist- 
ing of alternating basi- and inter-dorsal cartilages, generally 
supported by more or less well-developed chorda-centra. Pectoral 
and pelvic fins uniserial. Pelvic girdle and claspers present. 
Except in two families the branchial arches and clefts are 
invariably five in number. An operculum is not developed. 
Sub-Order 1. Selachii. 
Body elongate or fusiform, shading imperceptibly into a 
powerful swimming tail. Pectoral fins of moderate size, with 
contracted bases; not confluent with the sides of the head. 
Branchial clefts lateral in position. Vertebral centra generally 
asterospondylic or cyclospondylie. 
This sub-order includes such typical Elasmobranchs as the 
modern Sharks and Dog-Fishes as well as numerous fossil repre- 
sentatives ranging from the Carboniferous, and probably from 
still earlier periods, to the present day. 
Fam. 1. Notidanidae.— Body moderately elongate, the spine- 
less dorsal fin opposite the anal. Mouth ventral; nostrils ventral, 
near the extremity of the snout, without oro-nasal grooves. 
Branchial arches and clefts six or seven. Interbranchial septa 
devoid of marginal frills. Notochord persistent and continuous, 
partially constricted by simple chorda-centra, each consisting of two 
distinct rings, without either concentric or radial lamellae, except 
1 Giinther, Study of Fishes, Edin. 1880; British Mus. Cat. Fishes, viii. 1870 ; 
Miiller and Henle, Syst. Beschr. d. Plagiost. Berlin, 1841. Hasse, Natiirl. Syst. 
d. Elasmobr. Jena, 1879. Goode and Bean, Oceanic Ichthyology, Washington, 
1895. Jordan and Evermann, Fishes of North and Middle America, Washington, 
1896, Pt. i. Smith Woodward, Vertebrate Palaeontology, Cambridge, 1898 ; id. 
Brit. Mus. Cat. Foss. Fishes, i. 1889, ii. 1891 ; Zittel, op. cit. 
