FUs'S OF ELASMOBEANCHS. 457 



the dorsal postaxial angle of which a tall iliac process (il) extends dorsad. Behind 

 the root of this process is another («), which is the articular surface for the basal cartilage 

 of the ventral fin. From the outer dorsal and outer ventral parts of the ischio-pubic 

 cartilage two cartilaginous bars proceed preaxiad for some distance, and then meet, thus 

 enclosing a space (o), wliich may be called an " obturator foramen." 



The Pectoral Fin (Plate LXXIX. fig. 3). 



The skeleton of this fin presents only two basal cartilages, whereof the preaxiad one 

 (p), short and quadi-ate, must, I think, be deemed the projiteri/gium ^, while the other 

 much elongated one (m) must include the mesoptei^ygiimi as well as the metapterygium, 

 unless we consider the former to be absent. Annexed distally to the propterygium is 

 a triangular cartilage 2, which appears to consist of, or to represent, parts of the four 

 most preaxiad radials in one mass. 



At the distal end of the metapterygium is an hexagonal cartilage, whence no less 

 than fourteen radials diverge, coalescing La various ways so as to produce a curious kind 

 of mosaic pattern. Eleven radials are attached to the distal margin of the meta- 

 pterygium ; and two pairs of these coalesce together proximally for a short space. 

 Almost all the radials have a small cartilaginous segment appended to their distal end ; 

 and these small cartilages increase very gradually in size, from the one attached to 

 the triangular preaxiad cartilage, to that of the last metapterygial radial : thence they 

 decrease in size postaxiad with much greater rapidity. Minute extra cartilages are 

 intercalated between the apices of the last radial attached to the metapterygium and 

 those of the three or four next postaxiad — one between each. 



The proportion borne by the cartilaginous skeleton to the whole fin is small ; and it 

 projects distally more towards the postaxial than towards the preaxial limb-margin. 



Compared with the pectoral of Chimwra ^, the coalescence at the bases of the radials 

 is less, save near the distal end of the metapterygium, where it is more. The series of 

 small distal cartilages is less complete ; but this may be due to the defective condition of 

 the specimen at my disposal. 



POLYODON FOLIUM. 



Ventral Fin (Plate LXXVIII. fig. 8). 



The ventral fin of this fish exhibits a most interesting structure, although unfor- 

 tunately I have been able to examine only a dried specimen. The interest consists in 

 the fact that the cartilaginous skeleton is made up exclusively of elongated radials 



' It is also thus designated by Gegenbaur in Chimcera. 



^ This is referred to the mesopterygium by Gegenbaur in Chimwra ; but with such an interpretation I 

 cannot agree. It seems to me to have no warrant, and to tend to mislead. 

 ' As represented by Gegenbaur, ' Untersuchungen,' pi. ix. fig. 15. 



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