376 Bashford Dean Memorial Volume 



be seen that the pelvis is alike in the two sexes. In its basal, anterior and middle portions, 

 the skeleton of the pelvic fin of the male is much like that of the female. In the specimen 

 figured by Goodey there is a slight amount of fusion of radials at the extreme anterior 

 end. This fusion of radials does not appear m Braus's figure. 



Osburn (1907) described and figured the pelvis and the pelvic fin skeleton of a 225' 

 mm. embryo of Chlamydoselachus. The sex is not stated, but the condition of the most 

 posterior radials is intermediate between that characteristic of the adult female and that 

 shown in the male figured by Braus. Osburn noted that each pelvic girdle (lateral half 

 of the pelvis) is pierced by eight foramina for nerves, and serves as a basal for about half 

 of the radials of the fin. In the mesenchyme stage, the two girdles fuse at the mid-line, 

 and in the stage figured "the separation at the anterior end is not yet complete.'' This 

 "separation" presumably refers to the presence of a suture between the two cartilaginous 

 elements in the adult stage. In the fossil Chladoselache (according to Dean, 1909) there 

 are two quite separate pelvic girdles forming a pair, and in the fin skeleton the basals 

 consist of small rod-like elements Hke the radials (Text-figure 44). 



After reviewing the Hterature on the embryological development of the paired fins 

 of selachians, Regan (1906.2, p. 731) states: "The mode of development of the fin- 

 girdles is in favor of the hypothesis that they are outgrowths of the basipterygia, and the 

 latter may well have been formed from the coalescence of the originally separate basal 

 segments of the supporting cartilages, since in the median fins also these are segmented 

 off from continuous laminae." Osburn (1907, p. 188) also inclines to the view that the 

 origin of the girdles may be traced to the supporting elements of the fin. He compares 

 the pelvic girdle of Chlamydoselachus to the basals of unpaired fins. 



The Myxopterygia. — Posteriorly and medially, the skeleton of the pelvic fin in 

 the male is decidedly different from that of the female since it is enlarged and modified 

 to form the framework of the copulatory organ, the myxopterygium. The skeleton of the 

 myxopterygium or "clasper" has been described and figured separately by Giinther 

 (1887) and by Leigh-Sharpe (1926), whose figures are reproduced as my Text-figures 

 47 and 115a (the latter on p. 472). It has also been described and figured as a part of the 

 pelvic fin by Braus (1902) and by Goodey (1910.1) whose figures are reproduced as my 

 Text-figure 46 and Figure 21, plate V. The endoskeletal elements involved in the for- 

 mation of this organ are in Hne with the basals but are in serial relation with the radials. 

 They appear to be radials that are enlarged, elongated and otherwise diiferentiated. 

 In the several figures, there are minor differences in the radials associated with the one 

 that is most highly developed, and m Braus's specimen the skeleton of the myxopterygium 

 is not differentiated to the same degree as in the others. Possibly, Braus worked on 

 a specimen that was not fully mature. Leigh-Sharpe's description (1926, p. 312) of the 

 skeleton of the claspers, illustrated by his Fig. 5a (reproduced as my Text-figure 115a, 

 p. 472), is as follows: 



