716 DEVELOPMENT OF THE SKELETON 



Mivart's investigations and the majority of his views were 

 independent of Thacker's memoir ; but he acknowledges that he 

 has derived from Thacker the view that pelvic and pectoral 

 girdles, as well as the skeleton of the limbs, may have arisen 

 independently of the axial skeleton. 



The descriptive part of Mivart's paper contains an account 

 of the structure of a great variety of interesting and undescribed 

 types of paired and unpaired fins, mainly of Elasmobranchii. 

 The following is the summary given by Mivart of the conclu- 

 sions at which he has arrived 1 : 



" i. Two continuous lateral longitudinal folds were deve- 

 loped, similar to dorsal and ventral median longitudinal folds. 



" 2. Separate narrow solid supports (radials), in longitudinal 

 series, and with their long axes directed more or less outwards 

 at right angles with the long axis of the body, were developed 

 in varying extents in all these four longitudinal folds. 



" 3. The longitudinal folds became interrupted variously, 

 but so as to form two prominences on each side, i.e. the primi- 

 tive paired limbs. 



" 4. Each anterior paired limb increased in size more rapidly 

 than the posterior limb. 



" 5. The bases of the cartilaginous supports coalesced as 

 was needed, according to the respective practical needs of the 

 different separate portions of the longitudinal folds, i.e. the 

 respective needs of the several fins. 



" 6. Occasionally the dorsal radials coalesced (as in Noti- 

 danns, &c.) and sought centripetally (Pristis, &c.) adherence to 

 the skeletal axis. 



" 7. The radials of the hinder paired limb did so more con- 

 stantly, and ultimately prolonged themselves inwards by mesiad 

 growth from their coalesced base, till the piscine pelvic structure 

 arose, as, e.g., in Sqtiatina. 



" 8. The pectoral radials with increasing development also 

 coalesced proximally, and thence prolonging themselves inwards 

 to seek a point d'appui, shot dorsad and ventrad to obtain a 

 firm support, and at the same time to avoid the visceral cavity. 



skeleton of the limbs, with which we are here concerned, is undoubtedly not of this 

 nature. 



1 Loc. fit. p. 



