The Anatomy of Chlamydoselachus 



367 



Text-figure 34) it is composed of three concentric layers as follows : "The outermost of 

 these layers is relatively thin and consists of cartilage; within this cartilage is a second 

 and lighter broad area which appears to be made up of transverse fibers. Within this 

 second layer and bounding the notochord is a third layer of a white tissue. At regular 

 intervals the third layer forms septa which produce the regular constrictions in the 

 central part of the notochord. It will be observed that the septa are more pronounced 

 ventrally than dorsally, and that they pass intra-centrally." The development of the 

 sheath is discussed by Daniel (1934, p. 70). 



In a large female specimen of Chlamydoselachus described by Goodey (1910.1) the 

 first eleven vertebrae possess ring-like thickenings of the chordal sheath, which project 

 inward in such a manner as to constrict the notochord and make it appear somewhat 



Text-figure 34. 

 Sagittal section through sixth to eighth 

 segments of the vertebral column of 

 Heptanchus maculatus, showing struc- 

 ture of the chordal sheath. 



chd, notochord; iz, inner zone, mz, middle zone 



and oz, outer zone, of the notochordal sheath; 



nc, neural canal; s, septum constricting notochord. 



After Daniel, 1934, Fig. 52. 



like a string of beads (Text-figure 30). The soft notochordal tissue gradually becomes 

 obliterated from the intervertebral spaces as it approaches the skull, so that in the space 

 between the first centrum and the cranium soft tissue is not present at all (Goodey, 

 1910.1, p. 555). This is apparently not true of Carman's large specimen (Text-figure 29) 

 in which the notochord (ch) is nowhere completely interrupted by the constrictions of 

 the chordal sheath. Continuing my account of the cervical region in Coodey's large 

 specimen: Each constriction appears below a basidorsal, so that the constrictions are 

 intra vertebral. Each thickening of the chordal sheath possesses a calcification, as shown 

 by the deeply shaded areas in Text-figure 30, c. c, and in a median vertical longitudinal 

 section of a single vertebra these calcified areas appear like two Vs placed point-to-point. 

 Thus each centrum has the form of a short cylinder constricted round its middle. There 

 are no articular surfaces, nor even septa, separating any two successive centra; the 

 notochordal sheath is continuous and the intervertebral spaces are filled in by successive 

 bead-like segments of the notochord. The relations of the notochordal sheath are 

 shown somewhat better in a smaller and presumably younger specimen studied by 

 Coodey (1910.1, Fig. 9, pi. XLIII), in which some of the constrictions are not so well 

 developed and the calcifications are not complete. 



In the unusually long trunk region of Chlamydoselachus, the notochord (Text-figure 

 32) is almost uniform in diameter; nevertheless, according to Coodey, it shows slight but 

 unmistakable signs of segmentation. This segmentation is described by Goodey as follows : 



