SPIRAL VALVE 267 



A and C, and A and D. The individual variations are perhaps 

 even more remarkable, and appear to be quite independent of age 

 and sex. By way of example it may be mentioned that valves 

 approximating to one or other of those represented by C and D 

 occur in different individuals of Raia maculata of the same sex 

 and similar in size, even in young specimens not more than three 

 inches in length. 



As regards other Elasmobranchs, the common Dog-Fish 

 (Scyllium, caniculaY has a well-developed spiral valve disposed 

 in twelve coils, which structurally represents a more highly 

 developed example of the type D. The existence of considerable 

 individual variation is nevertheless indicated by the fact that 

 in one specimen examined the valve was intermediate between 

 and D, five of the eight cones projecting forwards and three 

 backwards. In a specimen of Notidanus sp.^ there were as many 

 as twenty coils, which in disposition were intermediate between 

 B and G, approximating, however, more nearly to B. In a 

 specimen of the Port Jackson Shark {ReterodontusY the valve 

 had eight coils, and in structure was also intermediate between 

 B and C, but approached more nearly to C. Some of the 

 Hammer-headed Sharks (e.g. Sphyrna malleusY possess a type of 

 spiral valve which differs considerably from any of those hitherto 

 described, and is termed a "scroll" valve (Fig. 159, E). The 

 attached edge of the valve pursues a straight longitudinal course, 

 or at any rate only describes a half turn and back again in 

 passing from the pyloric to the cloacal extremity of the gut. In 

 the middle of its course the width of the valve is about equal to 

 two-thirds of its length, but towards either extremity it gradually 

 diminishes until the free and attached margins meet. The valve 

 thus constituted is rolled upon itself from left to right, the 

 successive coils being comparable to a series of cylinders placed 

 one inside the other, and becoming gradually larger both in 

 length and diameter from within outwards. A similar valve is 

 present in some of the Carchariidae. 



In the Holocephali (e.g. Chimaera monstrosaY the valve 

 describes only three and a half coils, and is further remarkable 

 in that the attached margin, for a considerable portion of its 



1 Jeffery Parker, op. cit. pi. xi. Fig. 5. ^ Ibid. p. 58. 



' Ibid. p. 58. ^ Ibid. p. 59. 



^ Ibid. p. 58, pi. xi. Fig. 6. 



