244 VERTEBRATE ANIMALS. 
one of the chief sources from which zsimg/ass is prepared, and 
the roe is sold as a delicacy under the name of caviare. The 
place of the Sturgeons in North America is taken by the Paddle- 
fishes (Sfatularia). 
The group of Ganoids represented at the present day by the 
Sturgeons and Paddle-fishes was formerly represented by nume- 
rous remarkable fishes, which are most abundant in the system 
of rocks known to geologists as the “Old Red Sandstone.” 
The graphic descriptions of Hugh Miller have placed many of 
these fishes before us as living pictures, but space will not allow 
of any further notice of them here. One, however, of the more 
striking forms is figured below (fig. 130). 
Fig. 130.--Cephalaspis Lyellit, from tlre Old Red Sandstone of Scotland. 
ORDER V. ELASMOBRANCHII (Gr. e/asma, a thin plate; and 
bragchia, gills).—This order includes the Sharks and Rays, and 
is distinguished by the following characters: The skull and 
lower jaw are well developed, but the skull is not composed of 
distinct bones, and simply forms a kind of cartilaginous box. 
The vertebral column ‘is sometimes cartilaginous, sometimes 
composed of distinct vertebraz. The integumentary skeleton 
is in the form of Alacozd scales (fig. 122, c)—that is to say, of 
detached grains, tubercles, or plates. There are two pairs of 
fins, corresponding to the fore and hind limbs, and the ventral 
fins are placed far back, close to the anus. The heart consists 
of an auricle and ventricle; and the dudbus arteriosus is rhyth- 
mically contractile, is provided with a distinct coat of muscular 
fibres, and is furnished with several transverse rows of valves. 
The gills are fixed, and form a number of pouches, which open 
internally into the pharynx, and communicate with the outer 
