480 PROF. ST. GEORGE MIVART ON THE 
pterygium. No structure exists in them which can be certainly considered the relic of a 
primitively biserial limb, while their genesis from primarily distinct radials, significantly 
indicated by a comparison of the dorsal fin of Notidanus with its ventral fin on the one 
hand, and with the dorsal fins of such forms as Chiloscyllium on the other, seems all but 
demonstrated by the simple ventral fin of Polyodon. 
Thus, while I agree with Professor Humphrey as to the ventral and pectoral fins 
being skeletal developments in lateral body-folds, I believe him to be quite mistaken in 
regarding them as divaricated portions of the fold which forms the anal fin (the ventral 
fold). The paired fins are of different origin; and the ventrals have only assumed their 
approximately mid-ventral position through adaptive developmental changes. 
On the whole, I feel strongly persuaded that Vertebrate limbs have been formed as 
follows :— 
(1) Two continuous lateral longitudinal folds were developed, 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, 7. ¢. 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 longi- 
tudinal folds, 7. e. the respective needs of the several fins. 
(6) Occasionally the dorsal radials coalesced (as in Notidanus &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 the piscine pelvic structure arose, as, ¢. g.,in Squatina. 
(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. ‘Thus they came to abut dorsally against the axial skeleton, 
and to meet ventrally 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 cheiroptery- 
gial axis. 
(10) The distal end of the incipient cheiropterygium either preserved and 
enlarged preexisting cartilages or developed fresh ones to serve fresh needs, 
