WOODWARD : SHARKS OF THE CRETACEOUS PERIOD. 65 
Cephalic spines of Hybodont sharks, Sphenonchus of the older 
authors, have long been known from the English Wealden. The dis- 
covery of these affixed to the heads of which they were the armature 
was thus to be expected sooner or later. More remarkable, however, 
is the proof of the persistence of the notochord afforded both by the 
original of pi. I., fig. 1, and by a smaller fossil, partly shown in 
pL II., fig. 1. The genus Notidanus, it is true, has retained the primi- 
tive character of the axial skeleton of its trunk from the latter part 
of the Jurassic period to the present day ; but examples of such 
lengthened persistence are few, and the range of the typical noto- 
ehordal Hybodus from the Trias to the Upper Greensand is note- 
worthy. In Mr. Beckles' two fossils the series of elongated neural 
arches and spines (n.s.) is seen regularly arranged, though in part 
fragmentary ; and in the smaller specimen, of which a portion is 
shown in pi. II., fig. 1, there are also distinct remains of the haemal 
arches (h). All these cartilages are well calcified in tessera3, as also 
are the basals of the dorsal fins ; but the space between the neural 
and haemal elements (not.) is quite destitute of calcifications of any 
kind. The dorsal fin-spines are somewhat displaced, the first (dl) 
being overturned and accidentally thrust behind the second (d2). 
Each is shown to have been supported in the usual manner of a 
dorsal fin-spine by a great triangular basal cartilage (b.) extending from 
its inserted end the whole length of the fissure on the hinder margin. 
At the distal border of the basal cartilage of the anterior fin, five 
small radials (r.) also occur, gradually increasing in length towards the 
hinder border ; and there are traces of delicate filiform rays for the 
support of the fin-membrane. 
Genus Synechodus. 
[A. S. Woodward, Proc. Geol. Assoc., vol. x., 1888, p. 288, and Geol. 
Mag. (3) vol. v., 1888, p. 496], 
While it is now proved that a typical species of the genus 
Hybodus survived as late as the period of the Wealden, it is interest- 
ing to recall the fact of the occurrence even in the Chalk of another 
genus of Cestraciontidse exhibiting but a slight advance upon the 
typically Liassic Palceospinax. So far as known, Synechodus only 
differs from Palceospinax in the more numerous cusps of its teeth, 
