12 PROF. G. B. HOWES ON THE SKELETON AND [Jail. 18, 



establishment of the Elasmobranch fin, but in exactly the reverse 

 order'". 



The researches of Huxley and Balfour have proved that the 

 propterygium of Gegenbaur (figs. 9 and 10, pt.) represents, 

 throughout the Elasmobranch series, but one or more preaxial rays. 

 It is the most variable of the three basal elements of the Shark's 

 fin, and most observers are now agreed as to its morphological un- 

 importance. The above-named writers are further at one in their 

 estimate of the morphological value of the Elasmobranch meso- 

 and metapterygia {ms., int., figs. 9 and 10). That they disagree, 

 however, upon at least one vital issue is well known, and the 

 balance of opinion holds to-day that the solution of the ' archi- 

 pterygium ' question is to be sought in a reconciliation between 

 their views. 



Huxley has described and figured (19, p. 48) the maximum 

 development yet observed for the so-called propterygium of the 

 Ceratodus pectoral fin. That structure cannot be definitely re- 

 cognized in the pelvic fin. The determination of Huxley (19), 

 Balfour (1), and v. Rautenfeld (22), which regards the axis of the 

 Ceratodus fin as the mesopterygium, is too familiar to call for com- 

 ment here. It must suffice to state that I accept it in the main, if 

 not wholly, and assume for the present that the entire axis has the 

 value which Huxley first assigned to it. 



It is at this point necessary to discuss, more fully than heretofore, 

 the nature of the differences between the pectoral and pelvic fin- 

 skeletons of Ceratodus. Schneider has asserted (23, p. 521) that 

 " die Seitenstrahlen der dorsalen und ventralen Halfte der Flossen 

 sind ungleich," also that the " Seitenstrahlen der dorsalen Halfte 

 der einen Flosse entsprechen derjenigen der ventralen Halfte der 

 anderen." There is an undoubted tendency towards the assumption 

 of the condition which he thus formulates for Ceratodus, and it 

 seems to me probable that a common determining cause may have 

 led up to it and the condition realized in Frotopterus {cf. Schneider, 

 p. 524) ; but the definition no longer holds invariable for the former 

 animal, in view of the facts thus far adduced. I have already 

 stated that the presence of one preaxial paramere in connexion 

 with each mesomere is a constant character of the Ceratodus 

 pectoral fin, and I turn now to the distribution of the postaxial 

 rays. I have given on p. 7 the average distribution for eight 

 pectoral fins examined. The minimum observed was, taking the 

 mesomeres in order of succession from within outwards, 3.1.2.1, 

 the maximum 4.2.2.2. In no case have I observed five rays in 

 attachment with the second mesomere, as stated by Schneider. Of 

 the eight specimens examined, the second mesomeres of five bore 

 each three rays ; the third and fourth of seven each two ; and the 

 fiftii of six each one. It is thus certain that variation in the distri- 

 bution of the postaxial parameres (c/. fig. G) is, beyond doubt, far 



' Giiuther originally advanced a somewhat similar opinion (14, p. 534), 

 but he conceived of the process as having gone on along lines as yet incapable 

 of support. 



