In the Holocephali (e.g. _Chimaera monstrosa_)[248] the valve describes only three and a half coils, and is further remarkable in that the attached margin, for a considerable portion of its {268}extent, does not form a regular spiral but describes only a slightly sinuous course. Posteriorly, the valve is more normal, and consists of about two cones with their apices directed forwards. In the Dipnoi the spiral valve is well developed, and in _Neoceratodus_[249] describes nine coils, and in _Protopterus_[250] 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[251] 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 considerable distance from the anus. The evidence afforded by petrified faeces or "coprolites" proves that certain extinct Crossopterygii (e.g. _Macropoma_, _Megalichthys_), like their living representative, _Polypterus_, possessed a spiral valve.[252] In _Amia_ and _Lepidosteus_[253] 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_[254] 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 {269}the width of the valve so far exceed the semi-diameter of the intestine as, by forward or backward deflection, to give rise to the highly characteristic cones of Elasmobranchs and Dipnoi. In the more specialised Teleostomi (Teleostei) the spiral valve is wholly wanting, except perhaps as a vestigial structure in certain Clupeoids, as, for example, _Chirocentrus_,[255] and possibly also in some Salmonidae.[256]