Vol. 6o.j EDESTUS IN THE COAL-MEASUKES OE BRITAIN. 5 



of this side has been accidentally cut away, but the notch is not due 

 to this cause, for the fragment cut off is similarly, though not so 

 deeply, notched. Some splinters of bone near by may, however, 

 indicate that fracture took place before fossilization. 



The upper view shows the specimen to be nearly equilateral, so 

 nearly so indeed that the inequalities may be, and probably are, 

 due to distortion, or imperfection, in fossilization. But this want 

 of equality must not be lost sight of, for if such specimens be not 

 practically equilateral, we lose the chief evidence for the median 

 position of these ichthyodorulites in the fish to which they belonged. 



Measurements of Edestus from Smallthorne, in millimetres. 



Length of lower margin a-b, PL I. fig. 1 59'0 



Height of base &tf-c/ 120 



Thickness of base at f-g 10'5 



Height at A, PI. I, fig. 3 ll'O 



Depth of groove at h 5'5 



Lower edge of crown, a-c IS'0 



L'pper margin preserved, c-d 140 



Lower margin preserved, a-e 50 



Broken end of crown, d-e 80 



Broken end of crown, thickness 2*3 



Length of spur 7*0 



The shape of the base below the crown and the form of the 

 trough of the upper surface show clearly that this specimen is 

 one segment of a series, fitting one into the upper groove of 

 another, as in Edestus. And the close resemblance between this 

 fossil and Edestus minor leaves no room for doubting their generic 

 identity ; but the question of species is more troublesome. 



That our fossil is more nearly allied to Edestus minor than to 

 any other known species of the genus is evident ; but the want of 

 a perfect crown prevents a completely-satisfactory comparison. The 

 type-specimen of Edestus minor, described by Newberry, had a 

 very little of the lower part of the crown or base preserved ; but 

 the upper part is lanceolate, and is described as having double 

 denticles ; this is unlike the Smallthorne fossil, which has the 

 denticles in triple form. Hitchcock's original specimen (see 

 text-fig. 1, p. 2), which was subsequently referred to E. minor by 

 Newberry, has the crowns of the teeth rather more obtusely 

 lanceolate than in the type, and each crown is said to have a 

 lateral spur of enamel similar to that seen in the Smallthorne 

 specimen, but the denticles are double. So far as one can judge of 

 the form of a segment in E. minor, it is unlike this British fossil. 

 The much smaller specimen from Moscow, referred by Dr. Karpinski 

 to E. minor, has a narrower and more acuminate crown than either 

 of the American specimens, but shows the same slender spur of 

 enamel, and the denticles are said to be as in E, minor. The base 

 below the crown is very like the same part in the Smallthorne fossil ; 

 but, on close comparison, it will be seen that the lower edge of the 

 enamel forms nearly a straight line in the Russian tooth, while a 



