Not long after the publication of Thacker's paper, an important memoir was published by Mivart in the Transactions of this Society[483]. The object of the researches recorded in this paper was, as Mivart explains, to test how far the hard parts of the limbs and of the azygos fins may have arisen through centripetal chondrifications or calcifications, and so be genetically exoskeletal[484].
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 conclusions at which he has arrived[485]:—
"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, 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 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, e.g., in Squatina.