30 SIRENOID AND CROSSOPTERYGIAN GANOIDS. 



The above definition includes all the characters common to this species, but distinc- 

 tive from some other, which I have been able to discover. PI. V, figs. 4, 10, may be 

 taken as fairly typical examples. Figs. 5, 9, though at first sight similar to the other 

 two, seem to me to run into polymorphus. Hence arises some hesitation as to the dis- 

 tinctness of the present species, which I nevertheless retain until the case is clear. 

 C. parvus is not altogether unlike what Agassiz calls C. Phillipsii, from the Oolite of 

 Stonesfield, but it would be idle to attempt to identify or distinguish that species from 

 the figure, 1 or from the single tooth of Ceratodus from Stonesfield now in the British 

 Museum. C. serratus, from the Keuper of Argovie, 2 resembles it in its prominent ridges, 

 small size and well-defined internal angle. It would be interesting to know more of this 

 species, and in particular whether the six denticles are normally present in the teeth of 

 either jaw. Serratus, to judge from the single tooth figured, has the ridges longer and 

 more convergent than parvus. In the collection from Maledi, made by the Geological 

 Survey of India, are two teeth which remind us of C. parvus (PI. V, figs. 11, 12). They 

 bear much the same relation to C. Hislopianus which parvus does to polymorphs, or ser- 

 ratus to Kaupii. The occurrence in India, Germany, and England of two sets of fossil 

 teeth — a larger form in each case, with less defined ridges and internal angle, and a 

 smaller, with apparently a more regular outline, defined ridges and a tolerably constant 

 internal angle — suggests that we may have to deal with two states of growth, of perhaps 

 three nearly allied species. I have considered this view, but cannot as yet accept it. It 

 must be established, if it should ever be established at all, by the production of more 

 numerous connecting states than are as yet known to exist. 



Formation. — Rhaetic. 



Locality. — Aust Cliff, on the Severn. 



B rference.— Agassiz, 'Poissons Fossiles,' vol, iii, p. 132. 



[C. obtusus, Jy.~\ 



Agassiz describes the single imperfect tooth known in these terms : 

 " It is easy to distinguish this species of teeth from the preceding. Its surface is 

 flat and perfectly smooth ; the blunt denticles do not project above the surface of the 

 crown ; they are merely separated by depressions merging into the notches of the external 

 margin. The anterior [posterior] denticle is smaller than the three succeeding ones, 

 which are slightly inclined forwards [backwards] ; the last [first], which is not larger than 

 the lateral denticles, is itself transverse. The internal margin is irregular. While the 

 surface of this tooth is in general flat, its posterior [anterior] margin is much raised 

 above the anterior [posterior], which falls gradually, as fig. 20 shows." 



1 ' Poissons Fossiles,' atlas, t. iii, pi. xix, fig. 17. 

 3 lb., t. iii atlas, t. iii, pi. xix, fig. 18. 



