WOODWARD : SHARKS OF THE CRETACEOUS PERIOD 63 
cleft and not quite terminal, while the teeth are relatively large and 
arranged in several series simultaneously in function. Each tooth is 
conical or cuspidate, the crown more or less striated, with one prin- 
cipal elevation, and one or more lateral prominences on either side 
diminishing outw T ards ; the root is somewhat depressed, but not 
expanded in a plane at right angles to the cusps of the crown. The 
notochord must have been persistent, there being no traces of verte- 
brae ; and the neural and haemal arches of the axial skeleton do not 
appear to have been strengthened by calcified intercalary cartilages, 
such as exist in modern sharks. Ribs are distinct. The shagreen is 
sparse, consisting of small conical, radiately -grooved tubercles, some- 
times fused into groups of three ; and two large hook-shaped semi- 
barbed dermal spines, fixed on broad bases, occur above and behind 
the orbit. The dorsal fin-spines are marked with smooth longitudinal 
ridges and grooves, and bear a double series of downwardly curved 
booklets along the middle of the posterior face, Both spines are 
somewhat arched, but the anterior is longer and more slender than 
the posterior. 
Now, it is interesting to remark that teeth and spines identical 
with those of the genus Hybodus occur scattered through all Mesozoic 
formations as far upwards as the Gault and Upper Greensand. Hybo- 
dont teeth, indeed, though not the dorsal fin-spines, range as far as 
the Upper Chalk. It has thus for a long time seemed probable that 
typical members of the genus Hybodus lived throughout all the 
Mesozoic period, at least until the deposition of the Chalk. Hitherto, 
however, the actual proof of the circumstance has not been forth- 
coming, and the generic determination of the teeth and spines has 
necessarily remained provisional. On the present occasion the writer 
is able to announce that some of the long-desired evidence is at last 
available for discussion ; and a glance at the following description, 
illustrated by plate I., will suffice to show that the characters of the 
teeth and associated spines are not altogether misleading. 
Hybodus basanus, Egerton. 
1845. Sir P. Egerton. Quart. Journ. Geol. Soc, vol. i., p. 167, pi. 4. 
1889. A. S. Woodward. Cat. Foss. Fishes Brit. Mus., pt. i., p. 273, 
pi. 12, figs. 1-5. 
