HOLOSTEI 495 



caviare are prepared annually. Large quantities of isinglass are 

 obtained from the air-bladders, in the United States and in Eussia. 

 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. platyrhynclius) 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 Acipenseridse 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 

 (Eed 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, like the Eugnathidae and 



