268 FISHES CHAP. 



extent, does not form a regular spiral but describes only a slightly- 

 sinuous course. Posteriorly, the valve is more normal, and con- 

 sists of about two cones with their apices directed forwards. 



In the Dipnoi the spiral valve is well developed, and in Neo- 

 ceratodus^ describes nine coils, and in Protopterus"^ six or seven. 

 The structure of the valve in the latter Dipnoid resembles that 

 of Scyllium canicula, except for the smaller number of cones. 



In the more generalised Teleostomi the valve is best developed 

 in the Sturgeon (Acipenser) and in Polypterus. In the former' the 

 valve is restricted to the posterior half of the total length of the 

 intestine, often extending to within an inch of the anal aperture, 

 and describing in its backward course about seven or eight coils. 

 The width of the valve is about equal to the semi-diameter of the 

 intestine, and the thickened free margin forms a well-marked 

 axial columella, round which the cavity of the gut winds, as in 

 the type B, except that the spiral is a more open one. In 

 Polypterus the valve begins close to the solitary pyloric caecum, 

 and for some distance pursues a straight longitudinal course, but 

 eventually forms a few spiral coils, ceasing, however, at a con- 

 siderable distance from the anus. The evidence afforded by 

 petrified faeces or " coprolites " proves that certain extinct Cross- 

 opterygii (e.g. Macropoma, Megalichtliys), like their living repre- 

 sentative, Polypterus, possessed a spiral valve.* In Amia and 

 Lepidosteus^ the valve is almost vestigial, being restricted to the 

 terminal portion of the intestine, and is somewhat variable as to 

 the precise number of its coils. In Amia there are nearly four 

 coils, extending over 3 cm., that is less than a tenth of the total 

 length of the intestine, but in some specimens the coils do not 

 exceed two and a half or three in number. Lepidosteus ^ has a 

 still shorter valve which, in specimens of 7-10 cm. in length, 

 may not consist of more than three and a half coils, and in much 

 larger specimens may be reduced to less than two coils, a 

 variation which suggests that a reduction takes place in the 

 number of coils as the fish increases in age and size. The 

 structure of the valve in the three last-mentioned genera 

 resembles that described in Acipenser, and in none of them does 



^ Giinther, op. cU. p. 544. '' Newton Parker, op. cit. p. 141. 



" Maoallum, Joxirn. Anat. and Phys. xx. 1886, pp. 618, 619. 

 '' Owen, op. cit. p. 424. ' Macallum, I.e. 



' Balfour and Newton Parker, op. cit. p. 425. 



