Sub-Class II-CHONDEOPTERYGII * 



Skeleton cartilaginous or osseous. Heart with a contractile conus 

 arteriosus. Optic nerves united, or should they decussate, do so only slightly. 

 Intestines with a spiral valve. 



Among this sub-class, British waters possess representatives of two orders : — 



I. Ganoidei — having a single gill-opening on either side, and the gills more or 

 less free. 



II. Elasmobranchii — several gill- openings, as a rule, on either side, or on the 

 lower surface, with the gills externally, more or less attached to the skin. 



Oeder I— GANOIDEI. 



Skeleton cartilaginous or osseous. Gills mostly free, rarely having a slight 

 connection with the walls of the gill-cavity : a gill-cover and a single gill-slit 

 on either side. Median and paired fins: the posterior pair abdominal. Air- 

 bladder with a pneumatic duct. Ova small. Embryo may have external free- 

 gills. 



Sub-Ordee I— CHONDROSTEI. 



Skeleton notochordal : skull cartilaginous. Branchiostegals, when present, 

 few in number. Teeth generally absent, sometimes only found in the young 

 stage. Nostrils in front of the eyes with two openings. Caudal fin heterocercal 

 with fulcra. Integuments scaleless, or with dermal ossifications which are also 

 present on the head. 



Family I— ACIPENSERIDA 



Body sub-cylindrical and elongated. Branchiostegals absent. Snout conical 

 or produced. Mouth inferior, transverse and protractile. Teeth (except in the 

 very young) absent. Barbels four in a row on the ventral surface of the snout. 

 G-ill-membranes confluent and attached to the isthmus. Grills four, two accessory 

 gills ; a spiracle at the upper border of the gill-cover. Dorsal and anal fins 

 approximate to the caudal; vertical ones with a single row of fulcra. Air- 

 bladder large, simple and furnished with a pneumatic duct that communicates 

 with the dorsal wall of the sesophagus. 



In these fishes, the skeleton is partly cartilaginous and partly osseous. 

 Professor Kitchen-Parker (Nature, May 19, 1881, p. 71) observed that "the 

 sturgeons, as a group, cannot be said to be directly between any one family of the 

 Selachians and any one family of the bony Ganoids, yet on the whole that is their 



* Pal^ichthyes, Giinther, is equivalent to Chondropterygii as it has generally been 

 regarded, except that the aberrant Cyclostomata have been removed, an order which some systema- 

 tists consider ought hardly to be placed among the class Pisces, while they discard the headless 

 Leptocardi. Prof. Huxley read a paper at the Zoological Society of LondoD, March 20, 1883, on the 

 oviduct of the Common Smelt (Osmerus eperlanus), and took occasion to remark on the relations of 

 the Teleostean with the Ganoid Fishes. He pointed out the spinal valve in the intestines of the 

 Teleostean, Chirocentrus dorab, and came to the conclusion that the proposal to separate the 

 Elasmobranchs, Ganoids, and Dipnoans into a group, apart from and equivalent to the Teleosteans, 

 was inconsistent with the plainest anatomical relations of these fishes. 



