1910.] ANATOMY OF THE FRILLED SHARK. 561 



to the cranium, they give a rigidity to the supporting elements 

 which is of service no doubt in enabling the fish to cleave the 

 water. In the caudal region they meet the demand for increased 

 strength caused by the purchase which the caudal fin obtains 

 upon the water. 



Compared with Hexanchus the vertebral column of Chlamydo-^ 

 selachus must be regarded, I think, as showing more specialized 

 characters. In the former the notochord is simply constricted by 

 annular thickenings of the cartilaginous sheath, no calcifications 

 being present. The vertebral column of Heptanchus, however, 

 is on the whole more specialized than that of Ghlamydoselachus . 

 for although the double-cone arrangement is not so pronounced 

 at the anterior end as in Ghlamydoselachus, yet the notochord is 

 constricted vertebrally by a series of calcified rings which assume 

 more and more the form of double cones towards the tail. And, 

 moreover, in the caudal region secondary calcifications may give 

 rise to a number of short rays radiating out from the centre of 

 the double cones. 



VI. Pectoral Girdle and Fins. (Plate XLV. fig. 18.) 



Pectoral girdle. — I have nothing to add to the account of the 

 girdle given by Garman (10. pp. 13, 14), except to say that my 

 observations, made on three girdles, agree with his description 

 and figure, neglecting of course a few trifling details due no 

 doubt to individual variations. 



Pectoral fins. — In regard to the pectoral fins there are certain 

 rather important differences between the account and figure 

 given by Garman and the observations which I have made on 

 three pairs of fins. The articulation of the fin with the coracoid 

 is unibasal, and the articular surface of the mesopterygium is 

 about twice as large as that of the propterygium. 



The propterygium is small, triangular in outline, and has a 

 hollow articular surface. It carries no radials. 



The mesopterygium is moderately large, is irregular in shape, 

 resembling somewhat a truncated triangle, not being nearly so 

 triangular as that figured by Garman. It carries radials on its 

 posterior edge which show various degrees of fusion. 



The metapterygiam is an elongate, laterally compressed carti- 

 lage, carrying a large number of radials. It has a rounded 

 dorsal ridge and gradually curves outward, not being practically 

 straight as represented by Garman. Usually it is bisegmentah 

 but in this respect it presents some interesting variations, for in 

 two fins which I have examined it is trisegmental. In the former 

 condition the proximal segment is the longer one and is more 

 laterally compressed than the distal segment. The trisegmental 

 condition is represented in two ways ; first by the addition of a 

 small proximal segment articulating with the coracoid, similar to 

 that figured by Braus (3), and second, by the intercalation of a 

 short segment between the normal proximal and distal segments. 

 Proc. Zool. Soc.— 1910, No. XXXVI. 36 



