454 PROF. ST. GEORGE MIVART ON THE 
the nature of these eighteen cartilages? Are they homologous with the basal, the 
median, or the distal cartilages of the triserial dorsal fin-skeletons ? 
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 undifferen- 
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 enlarged 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. 
TRYGONORHINA FASCIATA. 
Dorsau Fin (Plate LX XVIII. 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. 
