Figure 6-62. Pelvic girdles and fins of choanate fishes as seen in ventral view; A, Eusfhenopferon; 

 B, Lafimerio; C, Profopferus. 



scapula" extending medially from its dorsal end. Chlainydo- 

 selachus also has this element. In many sharks such a piece 

 is lacking; the scapulocoracoid merely tapers to a point. The 

 scapulocoracoids are joined ventrally through a separate 

 piece of cartilage which has been compared with the ster- 

 num. The presence of this median piece cannot be detected 

 in the adult. 



The base of the fin is generally built around three ele- 

 ments: a propterygium, a mesopterygium, and a metapter- 

 ygium. From these numerous radials extend out. The varia- 

 tions in shark fins are multitudinous but generally there are 

 three basal units as in Squaliis. Occasionally only two are 

 present, the propterygium being absent (Heterodonlus). In 

 fossil sharks this "uniformity" is lost; Cladosetachus and 

 Cladodus appear to have only the metapterygium (Figure 

 6-67). 



A most peculiar fossil "shark" is Pleuracanthus (Figure 

 6-67), which has a suprascapular element and a biserial 

 archipterygial type of fin. A similar but much shorter and 

 rounder fin is observed in Chondrenchelys. 



The presence of both broad-based and concentrated fins 

 (the archipterygium) in sharks and shark-like fishes sug- 

 gests that there was considerable experimentation with a 

 newly developed structure and that in the sharks, as repre- 

 sented by the living fauna, only a single basic type has come 

 down to the present. 



The pectoral appendage of Hydrolagus (Figure 6-66) is 

 comparable to that of the shark. The scapulocoracoids are 

 more broadly joined ventrally and the base of the fin lacks 

 an evident mesopterygium. The radials are longer and 

 separated by gaps like those of an actinopterygian. This type 

 of appendage and girdle fit well into the fossil array sug- 



gesting that the chimaerid pattern is independent of that of 

 the present day sharks. 



Pelvic appendage The pelvic fins of the shark (Figure 

 6-66) are quite similar to what we have seen in other fishes. 

 The puboischial processes meet at the midline and fuse; 

 there is a slight iliac process anterior to the fin base. 

 In Chlamfdoselachus there is a broad medial plate between 

 the fins. In Squalus the radials are arranged at an angle along 

 the metapterygial element and behind the propterygial unit. 

 In males of Squalus, and sharks in general, there is a clasper 

 extending back from the metapterygial element (see Good- 

 rich, 1930, Figure 165). The pelvic fin, except for the 

 clasper, appears to represent a less highly developed fin than 

 the pectoral — it remains at a more primitive structural 

 level, like the pectoral fin of Hydrolagus. 



In terms of its pelvic appendage, Hydrolagus is quite dis- 

 tinctive and unmatched among the sharks or among other 

 fishes for that matter. The pelvic girdle is like the pectoral 

 girdle in having a large iliac process extending upward in 

 the lateral body musculature. There is a strong puboischial 

 process ligamentously joined to the opposite pelvic element 

 at the ventral midline. Attached to the puboischial process 

 is a clasper which is normally sheathed in a pouch anterior 

 to the base of the fin. This anterior clasper has several 

 hooked denticles on it. The fin itself articulates with a dis- 

 tinct process. At its base is a large element with pro- and 

 metapterygial processes. Articulating with the posterior end 

 of the metapterygial process is a posterior clasper involving 

 two units of the metapterygial stem; the more posterior of 

 these is forked. 



The presence of a posterior clasper in male sharks and 



BILATERAL APPENDAGES • 183 



