XVI 
INTRODUCTION. 
well-marked type of skull is definitely known to persist for long 
periods ; and in this connection the case of Squatina , ranging from 
the Jurassic to the present day, may be particularly mentioned. 
Still more interesting, however, is the light Palaeontology seems 
destined soon to shed upon the history of some of the more specialized 
skulls. The remarkable snout of the well-known Pristis seems to 
date back at least to the Eocene period, though even at this time 
there are apparently some forms not quite reaching the modern 
stage in which the lateral teeth are all implanted in firm cartilage 1 ; 
but in Cretaceous rocks no Selachian with so highly-specialized a 
rostrum has yet been found, and the discovery at Mount Lebanon 
of a sawfish 2 in which the rostral teeth are very small and only in 
part in contact with the cartilages is thus of extreme interest, and 
perhaps of considerable significance. 
The mandibular and hyoid arches are known in “ Diclymodus ” 
Cope, of the Permian ; in Pcdceospinax of the Lias ; in Hybodus of 
the Wealden; and in Synechodus of the Chalk. In all of these 
extinct types the hyomandibular element appears to be more 
slender than in the majority of living Selachians ; and in “ Didy- 
modus ” and Synechodus (p. 325) a postorbital facette has been 
observed upon the pterygo-quadrate cartilage, evident])' implying a 
direct articulation with the cranium, such as occurs in the adult of 
the surviving primitive genus Notidanus. The examples of Palceo- 
sjyinaoc and Hybodus noted in the following Catalogue seem to be 
sufficiently well preserved to demonstrate that no such arrangement 
obtained in these genera. 
A point of minor importance in connection with the mandible of 
one extinct genus is also worthy of note, on account of its yet 
awaiting satisfactory explanation. The Cestraciont genus Astera- 
canthus (p. 307) is provided with perhaps the most powerful dental 
armature of all the Sharks possessing crushing teeth; and most 
likely for this reason there is evidence of some unusual arrangement 
or development of the muscles by which the successful wielding and 
support of the j aws are ensured. On the outer side of the man¬ 
dibular cartilage 3 , slightly in advance of a point halfway between 
the condyle and the anterior extremity, there is a large protuberance 
or boss ; much of the external surface of the cartilage has a coarsely 
fibrous appearance, and at the position of this curious prominence 
1 Propristis (p. 76). 
3 Sclerorhynuhus (p. 76). 
3 See figure and description by the present writer in Ann. Mag. Nat. Hist. 
[6] vol. ii. (1888), p. 337, pi. xii. fig.l. 
