The Anatomy of Chlamydoselachus 369 



»v'os n«9 



/■ — y — -, — y — y. — :^^ ^ TV 111 



ll\/.S 



Text-figure 36. 



Terminal caudal portion of the vertebral column of Chlamydoselachus, 



showing heterospondyly. 



ch, notochord; hs, haemal spine; n 108 — n 112, neuromeres. 

 After Goodey, 1910.1, Fig. 17, pi. XLV 



chus occurs; but from a comparison of Text'figure 33, after Goodey, with Text-figure 48, 

 p. 378, after Garman, it appears to be in the region of the dorsal and anal fins. Here, the 

 condition of the notochord and of the chordal sheath (Text'figure 33) is similar to that 

 in the trunk region (Text'figure 32). In Heptanchus (Daniel, 1934, p. 48) the transition 

 occurs at about the fifty'sixth segment dorsally, and somewhat farther forward ventrally 

 (my Text'figure 35); this region lies dorsal to the base of the anal fin. 



In the main caudal region of a large female specimen of Chlamydoselachus described 

 by Goodey (1910.1), the diplospondylous condition is well established (my Text'figure 

 31). The constrictions of the chordal sheath are of two sizes, the larger more calcified 

 ones lying beneath the imperforate dorsals, and the smaller less calcified ones beneath 

 the perforate dorsals. The segmented appearance of the notochord is due in part to 

 constrictions by bands of cartilage. These bands are lateral extensions of the dorsal and 

 ventral arcualia (basidorsals and basiventrals) round the chordal sheath, forming bridges 

 that connect the dorsal and ventral cartilages from which they arise. These bridges 

 alternate with spaces in which the chordal sheath is naked. In the trunk region, homolo' 

 gous bands of cartilage occur but they are so thin that they are recognizable only in 

 microscopical sections. 



Toward the tip of the tail the differences in the sizes of the cyclospondylous centra 

 gradually become lost, the constrictions becoming equal in size along with the equali' 

 zation in the size of the perforate and imperforate basidorsals. This stage marks a near 

 approach to perfection in the expression of diplospondyly. 



In the extreme tip of the tail the vertebral column is a gradually tapering structure 

 (Text'figure 36) which remains segmented up to the very end. The arrangement of the 

 nerve foramina with relation to the number of dorsalia is such that Goodey characterizes 

 this region as "heterospondylic." In Heptanchus (Daniel, 1934, p. 49) the segments of 

 the tail are said to show ""an incomplete diplospondyly'' in the arches both above and 

 below the central column. No exception is made in regard to the extreme tip of the tail. 



