XXVI 
INTRODUCTION. 
strate that no such points of broad systematic importance can be 
found. 
Prof. Gill, for example, regards Notidanus as the type of a 
“ suborder Opistharthri,” on account of the articulation of the 
pterygo-quadrate cartilage with the postorbital region of the 
cranium. According to Dr. Gunther \ however, the recently dis¬ 
covered Chlamydoselache falls into the same family as Notidanus; 
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. It ought also to be added that even 
in Notidanus itself the postorbital articulation does not arise until 
late in the history of the embryo 2 ; while in the adults of such 
widely diverse genera as Fleur acanthus and Synechodus a precisely 
similar feature of specialization is to be observed. 
Prof. Gill’s “ suborder Proarthrt,” typified by the existing Ces- 
tracion, 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 ( Synechodus ), indeed, 
which cannot yet be separated from the Cestraciontidae, exhibits the 
postorbital articulation of the pterygo-quadrate, exactly as in Noti¬ 
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 Selachii into Sharks and Days very nearly corresponds to a 
grouping suggested by the structure of the vertebral centra. In 
the Days (Tectospondyli) a series of concentric laminae surrounds 
the primitive double-cone of each vertebral centrum; in the majo¬ 
rity of Sharks (Asterospondyli) the arrangement of the secondary 
laminae is such as to impart a stellate aspect to transverse sections 
of the centra. 
These features are distinctive to such an extent, that we venture 
to adopt the arrangement; and in this way it is possible to place 
the Pristiophoridae and Squatinidse in their apparently natural 
position in proximity to the Days. 
1 “Report on the Deep-Sea Fishes’’ (‘Challenger’ Reports, Zool. vol. xxii. 
1887), p. 2. 
2 T, H. Huxley, Proc. Zool. Soc. 1876, p. 44. 
