IX SPIRAL VALVE 265 
seems to depend on several factors, the effect of which, in different 
Elasmobranchs, is best studied in a series of valves of progressively 
higher differentiation.’ 
In a hypothetical simple type of valve, easily derivable from 
the more primitive type of Petromyzon, it may be conceived that, 
while not exceeding in width the semi-diameter of the intestine, 
the valve becomes disposed in several complete and more or less 
closely approximated spiral turns, the free edge of the valve being 
on the same level as its attached margin, and leaving an open 
axial canal along the centre of the gut. The nearest approach 
to this hypothetical type, which has been compared, not inaptly, 
to un escalier tournant sans noyau, is perhaps to be found in the 
Thresher-Shark (Alopecias vulpes). 
The structure of the more complicated spiral valves of other 
Elasmobranchs are well illustrated within the limits of the single 
genus Raia. 
In one specimen of Raia sp. (Fig. 159, A) the last four coils 
of the valve are similar to those of the hypothetical type, but the 
more anterior ones, owing to the greater width of the valve, 
which here exceeds the semi-diameter of the intestine, have their 
free margins deflected downwards, while that portion of the valve 
which forms the first half turn is coiled inwards upon itself, so as 
to form a hollow cone, open dorsally, and having its apex directed 
forwards. In other examples a further modification is intro- 
duced by the increasing width of the valve, which now, throughb- 
out its whole length, equals the semi-diameter of the intestine ; 
and by the formation of an axial columella by the thickened free 
edge of the valve, which is traversed by a central band of 
unstriped muscle, as well as by the intra-intestinal artery and 
vein, and takes the place of the central canal of the preceding 
types. The valve is, however, still regular, and its free margin 
remains on the same level as the corresponding portion of the 
attached edge. In other specimens, again, additional complica- 
tions are introduced by a still further increase in the width of 
the valve, which now exceeds, often considerably, the semi- 
diameter of the intestine, and the consequent deflection of the 
free edge of the valve either forwards or backwards (C and D). 
As shown in C the valve, in consequence of the backward deflec- 
tion of its free margin, presents the appearance of a nest of 
1 T. Jeffery Parker, Trans. Zool. Soc. xi. 1879, p. 49. 
