448 J. FRANK DANIEL 
column by Hasse (’79); and on the anatomy and development 
of the fins by Thacher (77); by Mivart (’79), by Gegenbaur 
(65), and by Balfour (’81). From exhaustive studies of the 
first group we have corrected our understanding of the nature 
of the skull, while from those of the third we have gained illumi- 
nating evidence as to the origin of paired limbs. 
While considerable attention has been given to parts of the 
skeleton of the heterodont sharks, yet, so far as I am aware, no 
approximately complete study of any member of this group has 
previously been made. This may be due, at least in part, to 
the fact that the heterodont sharks are located in widely sepa- 
rated regions, and, furthermore, that they are not abundant in 
the localities where they occur. Heterodontus is more or less 
difficult to obtain on the California coast, yet through the efforts 
of the Scripps Institution of the University of California I have 
been provided with numerous specimens of various ages. I 
wish here to re-express my thanks to that Institution for its 
assistance. 
While the head region of Heterodontus is specialized, as 
Gegenbaur maintained, yet it is by no means without generali- 
zation. In fact marked simplicity may be found, side by side 
with specialization. Such a case is seen in the generalized type 
of quadrato-mandibular joint, accompanied by a highly special- 
ized mode of ligamentous articulation. 
It would be generally conceded that the fin acer also is 
specialized, if one agree with Mivart’s conception of speciali- 
zation meaning concrescence. However this may be, it is cer- 
tain that, had Mivart known of the unpaired fins of Hetero- 
dontus francisci, his argument for similarity of plan between 
paired and unpaired fins would have been even more convincing. 
THE ENDOSKELETON 
In the following paper I shall discuss at some length the 
endoskeleton of Heterodontus francisci. In such a consideration 
the skeleton naturally is divided into: (1) The axial part, 
including the skull and spinal column; and (2) the appendicular 
skeleton, embracing fins and fin-girdles. 
