ANATOMY OF HETERODONTUS: ENDOSKELETON 469 
the two plates were fully separate both above and below. In 
Heterodontus francisci only six or seven vertebrae form the 
transition between rib-bearing and haemal vertebrae (see also 
Gegenbaur ’67, pl. 9, fig. 19, and Secérov ’11, pl. 1, fig. 5). 
Upon these centra and those following, the plates of the neural 
arches are also modified, being so arranged that to each myomere 
two basidorsal and two interdorsal plates occur. Such verte- 
brae are called diplospondylous. 
In the transitional vertebrae alternate basidorsals, which are 
odd in number, are perforated by the ventral roots of the spinal 
nerve (f.v.), and alternate interdorsals are usually separated by 
the foramina of the dorsal nerves (f.d.). On the thirty-seventh 
vertebra (fig. 16) the foramen for the dorsal root, however, 
migrates forward so that it perforates the interdorsal (inter- 
calary) plate. In some other cases the dorsal root foramina 
pass between the interdorsal and the basidorsal, producing a 
more or less pronounced niche in the anterior part of the basi- 
dorsal plate. In this transitional area, the suprabasidorsal 
(s.b.d.) above imperforate basidorsals is single where the fora- 
men completely separates these. Above each perforate basi- 
dorsal it is usually doubled. These occur in regular fashion, 
capping the interspaces between basidorsals and the interdorsals 
so that in the case of figure 16. they comprise three (pairs of) 
nodules to each myomere. 
It will be observed, further, that haemal arches corresponding 
to the imperforate and perforate segments may be determined 
by their shape without reference to the basidorsals in question. 
This will be made especially clear by reference to the fortieth 
and forty-first basiventrals. The ventral termini of these point 
in opposite directions, the fortieth pointing anteriorly, and hence 
corresponding to the imperforate basidorsal, and the forty-first 
posteriorly, and hence belonging to the same segment as the 
perforate basidorsal. In a later study of the caudal region we 
shall have reason to refer to the foramen formed between the 
haemal arch of the imperforate segment (anterior) and the haemal 
arch of the perforate segment (posterior). It is through this 
that the main segmental artery leaves the caudal aorta. 
