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 hody. These lateral folds, then, are the homo- 

 logues of Wolfl[ian ridges, in embryos of higher forms. Here, as in 

 the median fins, there were formed chondroid and finally cartila- 

 ginous 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. 



"Thehm.bs 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 them- 

 selves 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 consti- 

 tuting 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 

 Society \ 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 exoskeletal". 



Mivart's investigations and the majority of his views were inde- 

 pendent 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 hmbs, 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 ^ : — 



"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 



1 St. George Mivart, "On the Fins of Elasmobranchii," Zoological Trans 

 vol. X. 



2 Mivart used the term exoskeletal in an unusual and (as it appears to me) 

 inconvenient manner. The 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 Loo. cit. p. 480. 



