1888.] MORPHOLOGY OF SUPERNUMERARY PHALANGES. 509 



to regard the supernumerarv elements as primarily intercalary, and in 

 all probability derivative of the inter-phalanges (inter-articular 

 syndesmoses — the " Zvyischenscheibene " of Henke and Reyher, 6). 

 We accordingly accept, so far as it might bear upon primary differ- 

 entiation of the parts, Leboucq's declaration (13. p. 532) that 

 " alle Phalangen [m the Cetacea] wie gross ihre Zahl sein moge, 

 denselben morphologischen Wert haben." 



The condition of the limbs in the living Sirenia referred to {ante, 

 p. 507) suggests that the numerical increase of the phalanges may 

 have been associated with the loss of the ungues, and it is interesting 

 to reflect here that elongation by regular segmentation of the 

 cartilaginous rays of the paired fins of the Batoidei would, as com- 

 pared with those of the Selachii, appear to have been somewhat 

 similarly associated with the suppression of the horny fin-rays. The 

 condition of the parts in Squatina and Zygcena would seem to be 

 transitional in this respect. We put this forward as a mere sug- 

 gestion, deduced by argument from structural analogy. 



It would be exceedingly instructive, in the light of the preceding, 

 to ascertain if the syndesmosis-like pads of the toothed whales pass 

 through a synovial stage during development. Henke and Reyher's 

 observations already cited (p. 502) show that in their ' Zwischenschei- 

 ben' we have to deal with a derivative of the syndesmosis. Our atten- 

 tion was early arrested by the general similarity between the proximal 

 syndesmoses in the Hylids and Ranids and the knee-joint in the 

 higher Vertebrata. Comparison of the parts in the latter with those 

 of the former as represented by Nototrema ([>. 502) reveals a striking 

 uniformity between the two ; and, should it be found that the semi- 

 lunar cartilages are, with their ligaments, differentiations of a 

 syndesmosis, a complete reconsideration of the morphological value 

 of the former will be imperative. We have examined some 

 Mammalian embryos in respect to this, but we withhold, for the 

 present, further comment thereon. It is pertinent to recall here 

 Parker's assertion (15. p. 487) that in Aves (Apteryx) the rudiment 

 of the mesotarsal semilunar pad bears in its centre " a rounded 

 nodule of hyaline cartilage," which he takes to be " the represen- 

 tative of the centrale tarsi, an element not hitherto recognized in 

 birds." 



VI. CONCLUSIONS. 



1. That the supernumerary phalanx of the Anura is a true phalanx, 

 and, at the same time, structurally identical with the inter-phalangeal 

 syndesmosis of these and the other Amphibia, all transitions be- 

 tween the two being represented in adults of the living forms. 



2. That the syndesmoses and phalanges are differentiations of a 

 common blastema. 



3. That the supernumerary phalanx of the Anura is probably 



Proc. Zool. Soc— 1888, No. XXXV. 33 



