102 



FOSSIL FISHES OF THE ENGLISH CHALK. 



throe middle teeth are much larger than the first and fifth teeth. The maxilla in 

 the type specimen (PI. XXI, fig. 8, mx.) is incomplete in front, while it is broken 

 across and slightly displaced at its thin hinder end ; but the greater part of its 

 border is preserved. Its outer face is flattened, and only marked with a feeble 

 tuberculatum at its hinder end. The largest teeth, as usual, are fixed in the 

 convexity of the oral border in its anterior third, but the hinder teeth are also of 

 considerable size. 



The fragment of mandible figured by Agassiz (torn, cit., pi. xxv b, fig. o) 

 belongs to a larger fish than the type specimen, but is probably referable to the 

 same species. A still larger left mandibular ramus, reduced in outline in Text- 

 fig. 29, is less imperfect, and exhibits the contour of the dentary bone. The oral 



Fig. 29. Portheus mantelli, Newton; outline of left dentary, three eighths nat. size. — Lower Chalk; 

 Burham, Kent. Dibley Collection (B. M. no. P. 8252). 



border of this specimen is only slightly wavy, and its. anterior convexity bears the 

 usual relatively large teeth. 



Many vertebrae and portions of fin-rays large enough to have belonged to the 

 same fish as the largest jaws, have also been found in the English Chalk. A well- 

 preserved vertebral centrum, with the two characteristic lateral pits, is shown in 

 PL XXI, fig. 9. 



Horizons and Localities. — Turonian zone : Lewes. Larger specimens from 

 zones of Holaster subglobosus to Rhynehonella, cuvieri : Burham, Kent. Also from 

 undetermined zones : Warminster, Wiltshire ; Guildford, Surrey. 



2. Portheus daviesi, Newton. Text-figure 30. 



1877. Portheus daviesii, E. T. Newton, Quart. Journ. Geol. Soc, vol. xxxiii, p. 511, pi. xxii, fly. 13. 



Type. — Imperfect right maxilla, probably from zone of Holaster subglobosus ; 

 British Museum. 



