1066 THE THEORY OF TRANSFORMATIONS [ch. 



By an exaggeration of the same process we at once get an approxima- 

 tion to the form of one of the sharp-snouted, or longirostrine, 

 crocodiles, such as the genus Tomistoma; and, in the species figured, 

 the obUque position of the orbits, the arched contour of the occipital 

 border, and certain other characters suggest a certain amount of 

 curvature, such as I have represented in the diagram (Fig. 527, b), 

 on the part of the horizontal coordinates. In the still more elongated 

 skull of such a form as the Indian Gavial, the whole skull has under- 

 gone a great longitudinal extension, or, in other words, the ratio 

 of x/y is greatly diminished ; and this extension is not uniform, but 

 is at a maximum in the region of the nasal and maxillary bones. 



a b c 



Fig. 527, a, Crocodilus porosus; b, C. americanus; c, Notosuchus terrestris. 



This especially elongated region is at the same time narrowed in an 

 exceptional degree, and its excessive narrowing is represented by 

 a curvature, convex towards the median axis, on the- part of the 

 vertical ordinates. Let us take as a last illustration one of the 

 Mesozoic crocodiles, the little Notosuchus, from the Cretaceous for- 

 mation. This little crocodile is very different from our type in the 

 proportions of its skull. The region of the snout, in front of and 

 including the frontal bones, is greatly shortened; from constituting 

 fully two-thirds of the whole length of the skull in Crocodilus, it 

 now constitutes less than half, or, say, three-sevenths of the whole ; 

 and the whole skull, and especially its posterior part, is curiously 

 compact, broad, and squat. The orbit is unusually large. If in 



