243 FOSSIL FISHES OF THE ENGLISH CHALK. 



imperfect, is seen to occupy about half the length of the back of the trunk, and 

 its anterior rays are very long and stout, the length of the longest considerably 

 exceeding the depth of the trunk at its insertion. All the rays preserved have 

 a long undivided basal portion, above which they are crossed by well-spaced 

 articulations. Their total number cannot be ascertained, on account of the 

 destruction of the middle of the fin. The anal fin (a.) arises opposite the hinder 

 termination of the dorsal fin, and, so far as preserved, exhibits ten or eleven 

 slender rays, which are divided and crossed by well-spaced articulations in their 

 distal portion for more than half their length. The caudal fin (c.) is fragmentary, 

 but its rays are comparatively stout. The characteristic smooth ridge which marks 

 the course of the lateral line is conspicuous along the whole length of the trunk. 



Portheus sp. (p. 102). Text-figure 78. 



The shape of the maxilla in the type species of Portheus is known to have been 

 so variable, 1 that the specific determination of detached examples of this bone 

 proves to be uncertain. The fine large specimen shown of one half the natural 

 size in Text-fig. 78 cannot, therefore, receive a definite name, though it probably 

 belongs to the same species as the fragments of a large fish provisionally referred 

 to P. mantelli on p. 102. The oral border of this new specimen is sufficiently 

 well preserved to show that it is much less arched than in the typical maxilla 

 of P. mantelli. Its upper border is also evidently straighter; but its posterior end 

 is broken both above and behind, so that this part does not admit of comparison. 

 Several of the smooth teeth are seen, and those in the deep anterior third of the 

 bone are especially large. The outer face of the bone, where well preserved, is 

 nearly smooth. 



Portheus sp. (p. 103). Text-figure 79. 



The imperfect mandible shown of two thirds the natural size in Text-fig. 79, 

 is peculiar on account of the regular concavity of its oral border and the sharp 

 angulation of its symphysial end. The outer face of the anterior part of the 

 dentary (d.) is smooth and gently convex ; and its symphysial border slopes 

 rapidly downwards and backwards. The bone ends behind in a slight coronoid 

 process, and the posterior quarter of the ramus is formed by the relatively small 

 articulo-angular element (ag.), which is produced forwards as a tapering plate 

 beneath the posterior half of the dentary. The teeth are narrow, high, and 



1 A. Stewart, "Teleosts of the Upper Cretaceous," Univ. Geol. Surv. Kansas, vol. vi, Paleont., 

 pt. ii, 1900, p. 269, pis. xxxvii — xxxix. 



