1886.] AND HYOID ARCHES IN A CRETACEOUS SHARK. 221 
gradual, but then very abrupt, while anteriorly the rise is much 
more uniform and produces a markedly tapering outline. Quite at 
the front, the cartilage has the appearance of being more robust than 
is the case further back; but this is perhaps chiefly due to the 
infolding of the lower edge for the production of a trough for the 
membrane bearing the undeveloped teeth. 
On comparing this form of mandibular arch with the various 
modifications observed among living Selachians, it is at once 
evident that none agrees so closely as that of the two genera of 
Notidanide. Heptanchus and Hexanchus', indeed, exhibit an 
arrangement that differs in no essential particular from that just 
described in the Cretaceous Hybodont. In both cases there is not 
only a well-developed pterygo-trabecular process—homologous (as 
shown by Prof. Huxley *) with the pedicle of the tadpole’s suspen- 
sorium,—but also a distinct postorbital prominence and articulation, 
corresponding to the otic process in the tadpole*. The mode of 
articulation of the lower jaw is also nearly identical in each case ; 
and though the fossil is at present much crushed, it requires very 
little careful study to discover that the hollows for the muscles for 
raising the mandible were quite as deep in the Cretaceous Shark as 
they are in the living genera under comparison; the upper border 
of the quadrate region, however, is much less thickened than in the 
Notidanida and agrees more closely with that of ordinary Selachians. 
Tn the hyoid arch, the upper or hyomandibular element (figs. 1, 
2, hm, and fig. 4) is comparatively small and slender. Its length is 
0°037 m., and the cartilage is considerably arched and flattened 
in what appears to have been an antero-posterior direction. The 
proximal extremity is imperfect, but was evidently somewhat 
expanded at its articulation with the cranium; this end is also 
slightly twisted with respect to the axis of the rest of the element. 
Just below the bend, the cartilage appears contracted a little when 
viewed from behind, but soon expands again, forming a_ blunt 
tuberosity (¢) on the side nearest the pterygo-quadrate ; and from 
this point it finally becomes gradually narrowed until its termination 
in the imperfectly-displayed articulation for the cerato-hyal. 
The cerato-hyal (figs. 1,2, ch) is 0°048 m. in length, and is com- 
pletely shown on the left side of the fossil, though somewhat mutilated 
at the distal end ; the lower part, however, is well preserved on the 
right. The cartilage is considerably arched in the ordinary manner, 
and is much Jess robust towards its upper end than in the rest of its 
length. Compared with the hyomandibular, it is remarkably stout. 
A little below the proximal end it becomes comparatively large and 
' See figures by C. Gegenbaur, ‘“ Untersuchungen zur vergleichenden 
Anatomie der Wirbelthiere.—U1. Das Kopfskelet der Selachier,” pl. x. I am 
also indebted to the kindness of Mr. Howes and Mr. Martin Woodward for 
every facility for studying the beautiful preparations of Heptanchus, Cestracion, 
&e. in the Biological Laboratory of the Normal School of Science. 
* 1. H. Huxley, loc. cit. p. 40. 
* It is interesting to note that Prof. Cope’s Permian Selachian skulls already 
referred to also exhibit this character, 
