1887.] PAIRED FINS OF CERATODUS. 15 



marks off tlie propterygium from the mesopterygium in the adult 

 Cestracion. On a comparison of the two, 1 submit, with some 

 degree of confidence, the opinion that the postaxial lobe of the 

 second mesomere of Ceratodus (fi2;s. 5 and 6, tnt.) is the homologue 

 of the Elasraobranch metapterygium. Comparison of that lobe and 

 its attached rays (fig. 6) with the metapterygium of Cestracion 

 and its rays (fig. 9) reveals a striking similarity, even in detail, 

 between the two. Did the metapterygium of the Shark unite, 

 as does its propterygium, with the axial plate ms., it would be 

 difficult indeed to find a distinction between the first named and 

 that which, in Ceratodus, I claim as its homologue. 



The homology which I here seek to establish bears out, with 

 certain modifications, Balfour's view cited that " the fiu of Ceratodus 

 has been derived from a fin like that of the Elasmobianchs." That 

 observer first recognized (1, p. 668) that the metapterygium (his 

 basipterygium) is morphologically the most important and, phyloge- 

 netically, the most primitive of the basal elements ; while he suspected 

 (ibid.), but did not demonstrate the fact, that that structure is formed 

 by the coalescence of rays. Huxley had already asserted this belief, 

 in deahng with the metapterygium oi Notidanus (19, p. 50), which 

 he regarded as being " formed by the coalescence of the axial ends 

 of the postaxial rays" (presumably on the shortening of the fin 

 axis). Dohrn has recently substantiated the deductions of Thacher, 

 Mivart, and Balfour under this head, in having found that the meta- 

 pterygium is (8, p. 174), in both jiectoral and pelvic fins of the 

 Shark, like the basal bar of the median fins, made up of " unpaare 

 Knorpelstrahlen, die anfiiuglich oder jede Verbindung mit anderen 

 Skelettelementen bleiben." He reiterates the statement on p. 182 

 in the words " was als Basipterygium beschriebeu ist, stellt nur die 

 verschmolzenen, wei sehr nah an einander leigenden, Basen der 

 Flosseustrahlen dar und existirt nicht unabhangig von diesen." 



In face of the above facts, my view demands that a primary dis- 

 tinction shall be demonstrated between the second pectoral mesomere 

 in Ceratodus and that lobe which I hold to represent the meta- 

 pterygium. In Giinther's original specimen (fig. 8) the said lobe 

 was not represented in that which is now known to be its typical form, 

 while the rays (r) which are usually attached thereto were for the 

 most part independent. The proximal two of these appear to have 

 been somewhat smaller than usual, but it is highly interesting to 

 note that the two distal ones were uniting at their bases to form 

 a plate-like structure {mt.) which showed no signs of confluence 

 with the adjacent mesomere. This plate corresponds in its mode 

 of origin with the metapterygium (basipterygium of Balfour), as 

 defined by the above-named authors, and, in its relationships to the 

 rest of the fin-skeleton, with the lobe now under consideration. 

 I regard its condition as there represented to be indicative of the 

 primary independence which my interpretation necessitates. 



Giinther goes on to say (p. 532) that he found " lines of the 

 former divisions" of the cecond mesomere of this specimen pre- 

 served, in the shape of tracts of white connective tissue. Huxley 



