454 PEOF. ST. GEORGE MIVART ON THE 



the nature of these eighteen cartilages 1 Are thej' homologous with the basal, the 

 median, or the distal cartilages of the triserial dorsal fin-skeletons 1 



It is sufficient to compare their position, with reference to the fin they sustain and 

 the body, with the relation borne to the same parts by the basal cartilages of one or 

 all the other forms described, to see at once that they cannot be basal cartilages. They 

 must therefore answer to distal, or to both median and distal cartilages in an undifi'eren- 

 tiated condition. 



They are simple in form, neither coalescing, bifurcating, nor showing any plain 

 evidences (in the specimen examined) of segmentation. They increase gradually in 

 length postaxiad from the first to the eighth and ninth, and thence still more gradually 

 decrease. 



The basal cartilages of other dorsal fins must then be represented by the two large 

 cartilaginous plates which sustain these eighteen radials, 



The more preaxial of these two cartilages is much the larger, though it supports but 

 seven of the radials. It is subquadrangular in shape, and seems continuous proximally 

 with the cartilaginous axial skeleton. It is adjoined preaxially by the most postaxiad 

 of a series of vertically elongated more or less oblique cartilages, extending upwards 

 from the neural arches towards, and nearly to, the dorsal mid line of the body, which 

 vertically elongated cartilages have the appearance of so many enlai'ged neural spines. 



The more postaxial cartilage is much smaller than its predecessor, and is subtri- 

 angular in shape, with one angle ventrad to join the axial skeleton. Its dorsal margin is 

 nearly as elongated as the dorsal margin of the more preaxial cartilage, and it supports 

 eleven radials. The dorsal half of its oblique preaxial margin (which slopes ventrad 

 and postaxiad) joins the postaxial margin of the more preaxial cartilage ; but its ventral 

 half is separated from the preaxially sloping postaxial margin of the cartilage in front 

 by a considerable membranous interval, triangular in shape, with its most acute angle 

 dorsad. We have therefore in all three forms, Pristiophorus, Pristis, and Rhynchobatus, 

 a triangular interval between the bases of the two large basal cartilages of the dorsal 

 fin, which interval is either occupied by membrane or (as in Pristiophorus) by a small 

 separate cartilage. 



TRYGONOKHINA FASCIATA. 



DoESAL Fin (Plate LXXVIII. fig. 6). 



The cartilaginous skeleton of this fin extends but a very little way into the relatively 

 very large expanse formed by the fin-rays. It consists of the longitudinal series of 

 contiguous cartilages, supported upon two very large cartilaginous plates, which are 

 continuous ventrally with the subjacent axial skeleton. 



The distal cartilages are twelve in number, and increase slightly in length from the 

 most preaxial to the fifth, and thence slowly decrease. Their apices are rounded. 



