460 PROF. ST. GEORGE MIVART ON THE 



impeding the lateral flexure of the body in swimming, because the plane of 

 its antero-posterior longitudinal adherence must be at right angles to that 

 of the longitudinal adherence of the dorsal fin. 



(2) That generally the pectoral fins join the body at too low a level to enable them 



to abut directly upon the vertebral centra (or their representatives) or upon 

 (he neural arches, while the solid paraxial skeletal elements are very short in 

 Elasmobranchs. 



(3) That they could not be directly connected in a straight line, even obliquely, 



with the skeletal axis without interfering with the body-cavity of that 

 region. 



On all these accounts the pectoral fins, and the ventrals also, must (if they are to rest 

 on a solid support for their more or less obliquely up-and-down mode of flapping) have 

 a narrow connexion with a sustaining structure not directly continuous, in a straight 

 line, with the skeletal axis ; and these exigencies would account for the difference 

 existing between the mode of attachment of the pectoral fins generally and the mode of 

 attachment of the dorsal fin of Notidanus. 



But it may be said that the radial cartilages of the dorsal fin are really the prolon- 

 gations outwards of the axial skeleton. This is the teaching of Gegenbaur, who, as 

 we have seen, considers the dorsal radials to have been originally but productions of 

 the neural spines. 



The almost universal absence, however, of concordance or any definite numerical cor- 

 respondence between these elements and the subjacent vertebrae seems conclusive against 

 this view. We have (in Notidanus, Spinax, Aeanthias, Pristioj)horus, Pristis, and 

 Rhynchobates) found a series of forms which agree well with a process of coalescence 

 and centripetal extension, but which quite disagree with the opposite view ; for, 

 according to the latter view, we should have to suppose that the neural spines became 

 segmented, that they then enlarged and serially cohered down to their very bases, and 

 that subsequently such solid base became absorbed close to the vertebral column, while 

 remaining more or less coherent at a greater or less distance from it — a supposition 

 which seems to me a very unlikely one. 



It may be objected that in Pristiophorus, Pristis, and Squatina there are elongated 

 cartilages preaxial to the dorsal fin, rising up from the axial skeleton, and seeming at 

 the same time to be neural spines and serial homologues of the large dorsal fin-plates 

 of those genera. But are these parts really neural spines 1 In Squatina not only do 

 all or part of the basal plates of the dorsal fin seem discontinuous with the subjacent 

 skeletal axis, but the neural spine-like cartilages, situated yet more preaxially, also seem 

 separate from the axial skeleton on which they rest. Are these structures, then, 

 neural spines segmented off, or are they dorsal radials which have as yet imperfectly 

 cohered ■? The study of development can alone solve this problem ; but if it should 

 turn out that that even these cartilages are centripetal chondrifications, it would fully 



