XVIII HOLOSTEI 495 
caviare are prepared annually. Large quantities of isinglass are 
obtained from the air-bladders, in the United States and in Russia. 
The organ is split open and washed; the inner lining is then 
stripped off and the bladder dried as rough isinglass. 
The second genus, Scaphirhynchus, which includes the 
Shovel-nosed Sturgeons, differs from Acipenser in the long, 
flattened, and almost spatulate shape of the rostrum, the sup- 
pression of the spiracles, and the union of the longitudinal rows 
of scutes beneath the dorsal fin to form a scaly armature com- 
pletely investing the tail. The distribution of the genus affords 
an interesting parallel to that of the Polyodontidae. Of the 
four species, one (S. platyrhynchus) is common in the Mississippi 
valley and in the rivers of the Western and Southern States of 
North America, while the remaining species, also exclusively 
fresh-water, frequent the rivers of Tartary. 
The Acipenseridee are not known to occur earlier than the 
Tertiary. Scutes, pectoral spines and fragmentary bones, indis- 
tinguishable from the corresponding parts of existing species, 
have been recorded from the London Clay of the Isle of Sheppey 
(Lower Eocene), and from later Eocene deposits in the Isle of 
Wight and Hampshire; and also from the Pliocene of England 
(Red Crag of Suffolk) and Virginia. 
Order III. Holostei (Lepidosteoidei). 
The Holostei include a_ large ‘and somewhat heterogeneous 
assemblage of Fishes, most of which are now extinct. As a 
group they are by no means easy to define or delimit. Widely 
separated from the Chondrostei, there is little evidence of the 
existence of connecting links between the two groups, although 
in some respects the Catopteridae may be regarded as transi- 
tional. On the other side, however, the Holostei shade off 
almost imperceptibly into the Malacopterygian Teleostei. In 
different fossil and recent Holostei there may be traced the 
gradual acquisition of the more special Teleostean characters 
and the elimination of the more archaic features of their remote 
Teleostome ancestors; and in a general sense this may be taken 
as the key to the more salient attributes of the group. It is not 
suggested that all the families of Holostei are on the direct lines 
of Teleostean descent. Some families, ike the Eugnathidae and 
