OF THE PAIRED FINS OF ELASMOBRANCHS. 53 



toral 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 unde- 

 scribed types of paired and unpaired fins, mainly of Elasmo- 

 branchii. The following is the summary given by Mivartof 

 the conclusions at which he has arrived :^ 



"1. Two continuous lateral longitudinal folds were deve- 

 loped similar to dorsal and ventral median longitudinal 

 folds. 



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

 dinal 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 longitu- 

 dinal folds. 



"3. The longitudinal folds became interrupted, variously, 

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

 primitive 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- 

 danus, &c.) and sought centripetally (Pristis, &c.) adherence 

 to the skeletal axis. 



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

 constantly, and ultimately prolonged themselves inwards by 

 mesiad growth from their coalesced base, till tlie piscine 

 pelvic structure arose, as e.g. in Squatina. 



'* 8. The pectoral radials with increasing development 

 also coalesced proximally, and thence prolonging themselves 

 inwards to seek a. point (Tappui, shot dorsad and ventrad to 

 obtain a firm support and at the same time to avoid tlie 

 visceral cavity. Thus they came to abut dorsally against 

 the axial skeleton, and to meet ventraiiy together in the 

 middle line below. 



"9. The lateral fins, as they were applied to support the body 

 on the ground, became elongated, segmented, and narrowed, 

 so that probably the line of the propterygium, or possibly 

 that of the mesopterygium, became the cheiropterygial axis. 



" 10. The distal end of the incipient cheiropterygium 

 either preserved and enlarged pre-existing cartilages or deve- 

 loped fresh ones to serve fresh needs, and so grew into the 



' Loc. cit., p. 480. 



