1910.] ANATOMY OF THE FRILLED SHARK. 553 



spaces are filled with soft notochordal tissue, and there are no 

 secondary calcifications in these areas. 



For comparative purposes I have thought it worth while to 

 give a drawing (fig. 10) of a corresponding anterior region from 

 another and larger specimen. In this there are seen eleven 

 definitely calcified eyclospondylic centra, which gradually increase 

 in width as we proceed from left to right. Each one is in the 

 form of two Y's placed point to point, and, moreover, corresponds 

 exactly in position with a calcified band on the exterior of the 

 notochord. It is worthy of note also that the soft notochordal 

 tissue gradually becomes obliterated from the intervertebral 

 spaces as we approach the skull, so that in the space between the 

 first centrum a.ncl the cranium soft tissue is not present at all. 

 The larger, more definite, and regular calcifications of the centra 

 in the larger specimen are of considerable interest because they 

 seem to indicate — as will be shown in another region of the 

 vertebral column — that the extent to which calcification takes 

 place depends upon the age of the specimen, for apparently the 

 size depends upon the age. The older the specimen the more 

 definitely and regularly calcified are the regions where calcifica- 

 tion may occur. Garman mentions this region, and says that 

 there are vertebral constrictions which are somewhat calcified, 

 but he does not state how far this condition obtains, and his 

 figure of a longitudinal vertical section taken in this region is 

 very indefinite. The calcified areas are represented as being of 

 irregular shape, much more rounded than those which I have 

 found. They are also continuous with one another, whereas those 

 which I have found are quite discontinuous. 



Region 2. — The "trunk region" is the longest of all, and shows 

 the least differentiation of the notochord. The dorsalia are 

 represented by basidorsals and interdorsals, triangular in outline, 

 suprabasidorsals segmented off from the apices of the basidorsals 

 as small wedge-shaped pieces. The ventralia are represented 

 by basiventrals, somewhat rectangular in outline, and rounded 

 interventrals. The latter are comparatively small, and gradually 

 decrease in size as we proceed posteriorly. The notochord is of 

 uniform diameter, and shows slight but unmistakable signs of 

 segmentation ; each segment corresponding exactly with a basi- 

 dorsal above and a basiventral below. The segmentation is 

 shown by a difference in the appearance of the chordal sheath 

 along lines corresponding in position to the ends of the basi- 

 dorsals. At these points there appear to be narrow rings or 

 annulations of the notochord as shown in fig. 11. In a view of 

 the cut surface of a vertical longitudinal section of a portion 

 from this region, no apparent constrictions of the notochord are 

 found to correspond with the external segmentation of the 

 chordal sheath. The interior of the chord presents a fairly 

 uniform appearance, as was noted by Garman. If, however, a 

 horizontal longitudinal section be made of the notochord, a 

 regular sequence of constrictions of the chordal sheath is at once 



