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Show 1881.] OF THE PAIRED FINS OF ELASMOBRANCHS. 657 " As the dorsal and anal fins were specializations of the median folds of Amphioxus, so the paired fins were specializations of the two lateral folds which are supplementary to the median in completing the circuit of the body. These lateral folds, then, are the homo-logues of Wolffian ridges, in embryos of higher forms. Here, as in the median fins, there were formed chondroid and finally cartilaginous rods. These became at least twice segmented. The orad ones, with more or less concrescence proximally, were prolonged inwards. The cartilages spreading met in the middle line ; and a later extension of the cartilages dorsad completed the limb-girdle. "The limbs of the Protognathostomi consisted of a series of parallel articulated cartilaginous rays. They may have coalesced somewhat proximally and orad. In the ventral pair they had extended themselves mesiad until they had nearly or quite met and formed the hip-girdle ; they had not here extended themselves dorsad. In the pectoral limb the same state of things prevailed, but was carried a step further, namely, by the dorsal extension of the cartilage constituting the scapular portion, thus more nearly forming a ring or girdle." The most important point in Thacker's theories which I cannot accept is the derivation of the folds, of which the paired fins of the Vertebrata are supposed to be specializations, from the lateral folds of Amphioxus; and Thacker himself recognizes that this part of his theory stands on quite a different footing to the remainder. Not long after the publication of Thacker's paper, an important memoir was published by Mivart in the ' Transactions' of this Society1. 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 chondrifica-tions or calcifications, and so be genetically exoskeletal2. 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 arrived3:- " 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 i St. George Mivart, " O n the Fins of Elasmobranchii," Zoological Trans. 2" Mivart used tbe term exoskeletal in an unusual and (as it appears to me) inconvenient manner. Tbe term is usually applied to dermal skeletal structures ; but the skeleton of the limbs, with which we are here concerned, is undoubtedly not of this nature. 3 Loc. cit. p. 480. |