On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain, 329 
abdominal. Vertebral column generally heterocercal, the upper lobe of the caudal 
fin produced. Gills attached to the skin by the outer margin, with several inter- 
vening gill openings; rarely one external gill opening only. No gill cover. No air 
bladder. Two, three, or more series of valves in the conus arteriosus. Ova large 
and few in number, impregnated, and in some species developed, within an 
uterine cavity. Embryo with deciduous external gills. Males with intromittent 
organs attached to the ventral fins.”—(Giinther.) 
This order is divided into two sub-orders: Plagiostomata and Holocephala ; the 
first comprising the sbarks and rays; the second, the chimeras. 
Sub-order 1. PLAGrIosroMaATA. 
“From five to seven gill openings. Skull with a suspensorium and the palatal 
apparatus detached. ‘Teeth numerous.’ —(Giinther.) 
The Plagiostomata may be divided into two groups, the (A) Selachordei or sharks, 
whose body is elongate, more or less cylindrical, gradually passing into the tail ; 
gill openings lateral, 
The (B) Batoidei or rays, in which the body is depressed, and surrounded by 
immensely developed pectoral fins, forming a broad flat disc. Caudal portion more 
or less rapidly contracted to form the tail. Gull openings, five in number, are 
always placed on the abdominal aspect of the fish. 
The fish-remains found in the Mountain Limestone formations have hitherto not 
proved very numerous, nor have they been discovered in a great number of 
localities, considering the large area occupied by this group of rocks, its great 
vertical thickness, and the large extent to which it has been excavated for com- 
mercial purposes. The Limestone in the great majority of localities does not 
appear to contain any remains of fishes, and with the exception of the Armagh 
district and that of Bristol, other localities, including Wensleydale, Kendal, 
Derbyshire, and Oreton in Salop, have added few, either specimens or species, to. 
enrich the knowledge of the ichthyic fauna of that ancient period. 
The fish-remains hitherto found in the Carboniferous or Mountain Limestone, 
belong with few exceptions to the Placiostomata—the sharks and rays—and consist 
of an almost endless variety of teeth, and a large number of spines. A very slight 
consideration of the anatomical constitution of an existing shark will give an 
idea of the difficulty attending the determination of the species of the several 
fossils, and still more of the almost utter impossibility of reconstructing, on a sufficiently 
certain and scientific basis, anything approaching a correct idea of the form and 
parts of the extinet fish. In existing sharks the whole framework of the body is 
frequently cartilaginous. The skull and mandible are well developed, but are entirely 
composed of cartilage ; the pectoral and pelvic arches are the same ; the vertebree 
are in many fishes cartilaginous, in others a slight ring of bone is embedded in the 
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