94 PISCES. 



fragmentary specimens, of which the largest seem to indicate 

 a fish about six or seven metres in length. 



There is no evidence of fishes of the family Acipenseridae 

 below the Eocene Tertiary, where scutes and other fragments 

 indistinguishable from the corresponding parts of the recent 

 Acipenser occur. The earliest fossils of this character are from 

 the 'London Clay of Sheppey, Kent, and bear the provisional 

 name of Acipenser toliapicus. The Polyodontidas are also 

 represented in the Eocene by the genus Crossopholis, which 

 is found well-preserved in the Green Kiver Shales of Wyoming, 

 U.S.A. This fish is closely similar to the recent Polyodon, but 

 is remarkable for the squamation on the trunk. The scales are 

 small and thin, arranged in oblique series but not in contact ; 

 they are merely little grooved discs, each with several posterior 

 denticulations. The ridge-scales on the upper caudal lobe are 

 numerous. 



Sub-Order 2. Protospondyli. 



Among fossils, no links are known between the most highly 

 specialized Chondrostei and the representatives of the next 

 sub-order. The nearest approximation to the higher grade 

 appears to be made by the Catopteridse (see p. 87). Even 

 these fishes, however, are later in time (so far as known) than 

 the earliest Protospondyli, which are represented by Acentro- 

 phorus in the Upper Permian. In the present state of know- 

 ledge, therefore, the demarcation of the Chondrostei from the 

 Protospondyli is absolute. The latter are always characterized 

 by (i.) a hemi-heterocercal tail, (ii.) the supports of the dorsal 

 and anal fins in single series, equal in number to the apposed 

 dermal rays, (iii.) the presence of more than five baseosts in the 

 pectoral fins, (iv.) the want of baseosts in the pelvic fins, and 

 (v.) the want of infraclavicular plates. The degree of develop- 

 ment of the vertebral centra is variable, but even when they 

 are completed (e.g., in Amia) the pleurocentra and hypocentra 

 alternate and remain separate at least in part of the caudal 

 region. Most usually the pleurocentra and hypocentra are 

 alternating wedges (the rhachitomous type of ossification in 

 the notochordal sheath), well seen in Heterolepidotus, Caturus, 



