XXVI 
INTHODtrCTION. 
strato that no such points of broad systematic importance can bo 
found. 
I’rof. Gill, for example, regards Hotidanus as the typo of a 
“ suborder Opisiharthbi,” on account of the articulation of the 
ptery go-quadrate cartilage with the postorbital region of the 
cranium. According to Dr. Gunther ‘, however, the recently dis- 
covered ChJumydoselache falls into the same family as JS’utidanns ; 
and, from any point of view, it would certainly be impossible to 
relegate the two genera just mentioned to groups more widely 
separated than families. Nevertheless, in Chlamydoselache there 
is no articulation between the pterygo-quadrate cartilage and the 
cranium ; and the hyomandibular is as robust as in many types 
that would rank as modern. Tt ought also to bo added that even 
in Notidanus itself the postorbital articulation does not arise until 
late in the history of the embryo ’ ; while in the adults of such 
widely diverse genera as FUnracauthws and SynecJiodus a precisely 
similar feature of specialization is to bo observed. 
Prof. Gill's “ suborder PROARinar,” typified by the existing Ces- 
traclon, would doubtless prove equally inconsistent with facts, if the 
relations of the mandibular and hyoid arches in its extinct allies 
could be ascertained. One Cretaceous genus {Si/nechodus), indeed, 
which cannot yet bo separated from the Cestraciontidm, exhibits the 
postorbital articulation of the pterygo-quadrate, exactly as in Koti- 
danus. 
Turning to the axial skeleton of the trunk, the elaborate researches 
of Prof. Carl Hasse have provided ample materials for discussion. 
As already explained, the Professor points out that the division of 
the Sclachii into Sharks and Hays very nearly corresponds to a 
grouping suggested by the structure of the vertebral centra. In 
the Rays (Tectospondyli) a series of concentric laminse surrounds 
the primitive double-cone of each vertebral centrum ; in the majo- 
rity of Sharks (Asterospondyli) the arrangement of the secondary 
lamina) is such as to impart a stellate aspect to transverse sections 
of the contra. 
These features are distinctive to such an extent, that we venture 
to adopt the arrangement ; and in this wa}' it is possible to jilaco 
the Pristiophoridso and Squatinid® in their apparently natural 
position in proximity to the Rays. 
‘ “Eeport on the Deep-Sea Fishes” (‘ Challenger’ Eeports, Zool. vol. xxii. 
1887), p. 2. 
T. H. Huxley, Proc. Zool. Soo. 1876, p. 44. 
