CHORD ATA. 153 



Class II. Pisces. Fishes. 



Cold-blooded forms, with paired fins and median fin-rays, branchial 

 respiration, and heart of one auricle and one ventricle. Mostly with 

 scales. 



Order 1. Selachii (Elasmobranchii, Plagiostomi). Cartilaginous fishes 

 with placoid scales (dermal denticles), large pectoral and pelvic fins 

 (the former tribasal), five pairs of gill-pouches (usually), a muscular conus 

 arteriosus containing rows of valves, a spiral valve in the intestine, and 

 a cloaca. 



Squalidse (Selachoidei), sharks, with lateral gill-clefts. Scyllium. 

 Cestracion. Mustelus laevis (smooth shark of Aristotle). Acanthias 

 vulgaris. Notidanus (Heptanchus), seven pairs of gill-pouches. To 

 these are appended the Rajidae (Batoidei), with ventral gill-clefts. 

 Pristis. Torpedo marmorata. Narcine. Raja clavata, R. batis. 



Order 2. Holocephali. Closely allied to Selachii, but skin naked, upper 

 jaw fused to skull, notochord persistent, and four gill-clefts covered by a 

 membranous operculum. No cloaca. Chimsera, Callorhynchus. 



Order 3. Ganoidei. Cartilaginous and bony fishes with scales or 

 scutes (ganoid scales), comb-shaped gills covered by a bony operculum, 

 muscular conus arteriosus containing rows of valves, and a spiral valve in 

 the intestine. No cloaca, but a urogenital aperture behind the anus. 

 Numerous fossil, few recent forms. 



Cartilaginous Ganoids, with persistent unconstricted noto- 

 chord, heterocercal tail, spiracles and mandibular pseudobranchs. Aci- 

 penser, sturgeon, Scaphirhynchus, and Polyodon (Spatularia), recent. 

 Palaeoniscus Freieslebeni, Kupferschiefer. Platysomus. 



Bony Ganoids, with notochord constricted by vertebral centra, 

 skull with membrane bones, and no mandibular pseudobranchs. Polyp- 

 terus bichir, double swim-bladder, and Oalamoichtliys, recent. Ho- 

 loptychius, Devonian. Amia, Cretaceous to recent, A. calva, recent. 

 Lepidosteus osseus, bony pike, recent. Pycnodus, the unconstricted 

 notochord surrounded by demi- vertebra, Jurassic to Tertiary. Lepidotus 

 and Amblypterus in the Kupferschiefer. 



The armoured Ganoids Coccosteus, Pterichthys, and Cephalaspis 

 Lyellii are peculiar extinct forms from the Devonian. 



