1910.] ANATOMY OF THE FRILLED SHARK. 557 



It can be seen from fig. 13 that in this diplospondylic region we 

 have an alternation of imperforate and perforate basidorsals, 

 between which occur the perforate and imperforate interdorsals. 

 On the ventral side of the notochord we have a similar segmen- 

 tation of the ventralia. The notochord in this region has a 

 segmented appearance, which is brought about by the occurrence 

 of bands of cartilage round it. These bands are in reality 

 extensions of the dorsal and ventral arcualia — basidorsals and 

 basiventrals — round the chordal sheath, and they alternate 

 regularly with spaces in which the sheath is naked. They are 

 found in the trunk region as well, only there each cartilaginous 

 band is very thin, and is only recognizable in microscopic sections. 



In the main caudal region, however, the bands are much more 

 pronounced in growth. Here also there is a marked difference 

 in the relative size of the two kinds of basidorsals. The imper- 

 forate ones are larger than the perforate, and this difference in 

 size obtains especially where the caudal fin is deepest. As we 

 proceed towards the tip of the tail the dorsalia gradually become 

 more nearly equal in size until at a point just anterior to where 

 they lose their identity, they are quite equal. 



Perhaps the most interesting feature, however, of the skeleton 

 in this particular region is that which is found from a point level 

 with the posterior end of the anal fin to within a short distance 

 of the tip of the tail. Here we find definite calcified rings round 

 the chordal sheath which correspond in position to the basidorsals 

 above and lie internal to the cartilaginous bands just mentioned. 

 They are shown in fig. 13, where they appear as unshaded bands 

 on the notochord extending between a large imperforate basi- 

 dorsal above and the corresponding basiventral below. There 

 is also an indication in the figure of a band beneath a perforate 

 basidorsal and this, as it stands, may be somewhat misleading, 

 appearing as if it were somewhat exceptional. This, however, 

 is not the case, as only a short distance posterior to the portion 

 figured the calcified rings become as well marked beneath the 

 perforate basidorsals as beneath the imperforate ones. Fig. 14 

 represents a longitudinal vertical section of fig. 13. It is at 

 once apparent from this that internal to each broad calcified 

 band, i. e. beneath each imperforate basidorsal, we have a con- 

 striction of the chordal sheath in the form of an incipient centrum, 

 the calcification extending into it and lending it additional 

 strength. Beneath each perforate basidorsal also there is a veiy 

 slight constriction of the chordal sheath without any trace of 

 calcification. 



The points just dealt with are shown much more clearly in a 

 portion of the vertebral column taken from the larger, and 

 probably older, female specimen. In this the extensions of the 

 arcualia are very pronouncedly developed in the main caudal 

 region, those beneath the perforate basidorsals being quite well 

 shown. Both the latter and the larger ones beneath imperforate 

 basidorsals are ridded in surface view as shown in fiff. 15 In a 



