INTRODUCTION. 
XVII 
these fibres are directed in such a manner as to suggest the original 
influence of some powerful strain tending in an upward and back¬ 
ward direction. 
The branchial arches have only been definitely revealed in one 
extinct Selachian —Hybodus hasanus , from the Wealden of Sussex. 
In this species, and hence presumably in all forms of Hybodus , there 
are not more than five arches, as well shown in the original of 
PI. XII. fig. 3, in which the fourth and fifth are very small, and 
can scarcely have been followed by others. 
Vertebral Column. 
Before the end of Palaeozoic times there is very little evidence of 
calcifications in the sheath of the notochord in Elasmobranchs ; and 
even when such a stage of development is approached there seems 
to be no constriction. Hasse has noticed the presence of complete 
calcified rings in the caudal region, of a Permian species of Pleura- 
canthus (p. 4) ; and Traquair records an equally high condition of 
development in the tail of the Lower Carboniferous Chrondren- 
chelys (p. 15). Some specimens of Pleuracanthus are also sugges¬ 
tive of the presence of distinct triangular calcifications in the 
notochordal sheath in the abdominal region ; but it is still un¬ 
certain whether these may not be merely the expanded bases of the 
neural arches. 
The early species of the genus Hybodus , discovered in the Lower 
Lias, are also destitute of vertebrae, at least in the abdominal region ; 
and it is especially interesting to observe an almost equally primitive 
condition of the neural arches and spines (PL YII. fig. 2). The 
latter are relatively broader and stouter than in the Palaeozoic 
Pleuracanths, but there is still not the slightest trace of the inter¬ 
calary cartilages so characteristic of modern Selachians ; and this 
circumstance becomes all the more noteworthy when it is remem¬ 
bered that, among living Sharks, the intercalary elements are 
secondary structures, arising subsequently to the normal parts of 
the vertebral axis. 
The first traces of completed vertebral centra are met with in the 
Cestraciont Palceospinax of the Lower Lias—a fish exhibiting other 
features denoting its comparatively high degree of specialization. 
Here, however, the centra are for the most part simple double 
cones, such as persist in the living Spinacidae, and only the faintest 
indications of the secondarily developed peripheral calcifications can 
b 
