FmS or ELASMOBEANCHS. 441 



They seem more segmented than in Zygmna, and irregularly so ; but this may be due 

 to accidental fractures produced in preparing the dry specimen examined. 



The Caudal Fin (Plate LXXV. fig. 1). 

 The skeleton of this fin is remarkable for the extreme difi'erence 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 well as long. Notwithstanding this change of form and proportion, 

 the ventral-fin cartilages bear an almost exact numerical relation to the vertebrae — 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. 



DoKSAL Fin (Plate LXXIV. 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 lasal 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. 



The Caudal 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 vertebrae, there being one haemal cartilage to each vertebra. 

 The seven or eight most preaxial haemal cartilages are distinctly segmented off (with a 

 line of fibrous tissue indicating the separation) from the superincumbent parapophysial 

 vertebral elements (j))- The more postaxially situated haemal cartilages, however, are 

 without any such segmentation. 



3o2 



