368 Bashford Dean Memorial Volume 



The segmentation is shown by a difference in the appearance of the chordal sheath 

 along hnes corresponding in position to the ends of the basidorsals. At these points there 

 appear to be narrow rings or annulations of the notochord as shown in Fig. 11 [my Text-figure 

 32]. 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 apparent. 

 Each of these occurs beneath a basidorsal, and extends between two consecutive segmen- 

 tation marks on the exterior of the chordal sheath. Each takes the form of a bulging inward 

 of the sheath, so that a slightly pinched-in cylinder is formed. 



There are no calcifications of the notochordal sheath in the trunk region of Chlamy 

 doselachus. Rudimentary ribs are shown in Text'figure 32. 



The cervical and trunk regions are typically monospondylous, i. e., each ""neuromere" 

 (Goodey's terminology) is made up of one of each kind of vertebral element: basidorsal, 

 interdorsal, supra-basidorsal, basiventral and interventral. The foramina for the spinal 

 nerves do not occur between the dorsalia but are actual perforations of the basidorsals 

 and interdorsals. In the monospondylous regions each basidorsal transmits a foramen 

 for a ventral root, and each interdorsal, one for a dorsal root. At the seventieth neuro' 

 mere, Goodey found an interesting transition from the monospondylous to the diplo' 

 spondylous condition (my Text-figure 33). There is a doubling of the number of basidor' 

 sals, interdorsals and supra-basidorsals, but only the posterior interdorsals and basidorsals 

 of each neuromere contain foramina for the exit of the roots of spinal nerves. In the 

 seventy-second neuromere the monospondylous condition recurs dorsally, but the 

 ventral elements are diplospondylous. The diplospondylous condition characteristic 

 of the caudal portion of the vertebral column in sharks probably arises out of the monO' 

 spondylous by a process of fragmentation of the primitive cartilaginous vertebral 

 elements. 



Goodey does not tell us precisely where, with reference to external features, the 

 transition from the monospondylous to the diplospondylous condition in Chlamydosela- 



ja 50 "it f.v.f.d. to 



Text-figure 35. 



Lateral view of the spinal column of Heptanchus maculatus in the region of transition from the 



monospondylous to the diplospondylous condition, near the base of the anal fin. 



chd., notochord; f.d., foramen for dorsal nerve root; /.«., foramen for ventral nerve root; h.a., haemal arch; r., rib; 



s., septum constricting notochord; 44-60, vertebrae. 



After Daniel, 1934, Fig. 53. 



