46 
ACTIXOPTEKYGrII. 
Acipenser sp., A. S. Woodward, loc . cit. p. 29.—Bed Crag; 
Suffolk. [Pectoral spines ; York Museum.] 
The dermal tubercles named Acipenser tuberculosus and A. moilas- 
sicus by Probst are Selachian armour (see Part I. p. 89); and the 
so-called Acipenser cretaceus (A. Daimeries, Bull. Soc. Boy. Malacol. 
Belg. vol. xxvii. 1892, p. xvi) from the Senonian of Folx-les-Caves, 
Belgium, seems to be founded on the dermal plates of some fish 
related to Dercctis or Eurypholis. 
Family POLYODONTIDAE. 
Trunk elongate or elongate-fusiform; tail heterocercal. Bones of 
the cranial roof forming a discontinuous shield, with vacuities and 
with a median longitudinal series of small azygous elements on the 
rostral region ; snout much produced and eye far forwards; mouth 
large, with minute teeth in both jaws throughout life; no pre¬ 
maxilla ; operculum and suboperculum present, but no branchio- 
stegal rays. Squamation of trunk rudimentary or absent; the 
lateral rhombic scales and large fulcral scales on the upper caudal 
lobe robust. 
Only two genera of this family are recognized among existing 
fishes, namely, Polyodon (or Spatularia) from the Mississippi, and 
Psephurus from Chinese rivers. Fossil remains are very rare, and 
only one definable extinct genus has hitherto been discovered. A 
fragmentary specimen, however, from the English Chalk may per¬ 
haps represent another member of the family. 
A detailed description of the skeleton of Polyodon is given by 
T. W. Bridge in Phil. Trans. 1878, pp. 683-733, pis. lv.-lvii. A 
figure of the skull is also published by B. H. Traquair, Ganoid 
Fishes Brit. Carb. Form. (Palseont. Soc. 1877), pi. vii. fig. 1. 
Genus CROSSOPHOLIS, Cope. 
[Amer. Nat. 1883, p. 1152 (Crassopholis).'] 
External bones unornamented ; rostrum with small stellate bones. 
Dorsal and anal fins short-based and remote, the former arising in 
advance of the latter; caudal fin inequilobate, the upper lobe pre¬ 
dominant. Scales of trunk small and thin, each in the form of a 
small grooved disc with several posterior denticulations, arranged 
in oblique series, but not in contact; caudal fulcral scales numerous, 
broad at the base of the lobe, but becoming slender in its distal 
portion. 
