FINS OF ELASMOBRANCHS. 44] 
They seem more segmented than in Zygena, and irregularly so; but this may be due 
to accidental fractures produced in preparing the dry specimen examined. 
THE Caupa Fin (Plate LXXYV. fig. 1). 
The skeleton of this fin is remarkable for the extreme difference between its ventral 
and dorsal portions. The dorsal supports are very small and inconspicuous, while the 
ventral ones are large, and vary extremely in size, the ninth to the thirteenth being 
exceedingly broad as wellaslong. Notwithstanding this change of form and proportion, 
the ventral-fin cartilages bear an almost exact numerical relation to the vertebree—one to 
each—save that in passing postaxiad from the tenth vertebra, one seems to disappear ; 
but it is possible this may be due to the drying and preparation of the specimen. 
MUSTELUS ANTARCTICUS. 
Dorsa Fin (Plate LX XIV. fig. 3). 
The cartilaginous radials in the dorsal fin of this species form a triple longitudinal 
series of unequal number, without bifurcation, and with hardly any coalescence. The 
whole cartilaginous fin-skeleton is separated from the subjacent axial cartilages by an 
interposed fibrous membrane, which at its narrowest part is a little wider (dorso-ventrally ) 
than the length of the longest of the basal series of cartilages. 
The basal cartilages are eighteen in number. They increase gradually in length 
from the first to the eighth and ninth, and then more rapidly diminish, the four last 
being shorter than the first. 
The median cartilages are twenty-four, and for the most part are articulated to the 
dorsal ends of the basal cartilages, one to each. But the first three and the last three 
median cartilages have no such support, and are only connected ventrally with the 
supravertebral fibrous membrane. ‘The two most postaxial of these cartilages coalesce 
together at their ventral ends. 
There are twenty distal cartilages attached to the distal ends of the twenty 
posterior median cartilages, one to each. They increase somewhat in length and 
thickness to the fifth, and then decrease gradually in length, but not in thickness, to 
the fifteenth, and then become considerably longer till the last, which is a little shorter 
than the first. 
Tue Caupa Fin (Plate LXXIV. fig. 6). 
This fin is supported by a number of cartilaginous rays, which on the ventral side are 
equal in number with the vertebree, there being one hemal cartilage to each vertebra. 
The seven or eight most preaxial hemal cartilages are distinctly segmented off (with a 
line of fibrous tissue indicating the separation) from the superincumbent parapophysial 
vertebral elements (p). The more postaxially situated hemal cartilages, however, are 
without any such segmentation. 
302 
