182 FOSSIL FISHES OF THE ENGLISH CHALK. 
uniform in diameter. Some of 
those at the base of the caudal fin are shown in fig. 
10a, while the thickened portion of a caudal scale of the lateral line is added in 
fig. 106. In these cases, as in others, it is evident that the median tubercles are 
not relatively large and prominent. A specimen obtained by Mr. G. E. Dibley 
from Burham shows that the lateral line is marked by the peculiarly thickened 
scales along the whole length of the trunk. 
Horizon and Locality.—Zone of Schloenbachia varians : Folkestone and Dover, 
Kent. Zone of Holaster subglobosus : Burham, Kent (G. EH. Dibley). 
Subclass HOLOCEKPHALT. 
Order CHIMMROIDET., 
Family CiiMmRipm. 
The remains of Chimeroids hitherto discovered in the English Chalk are very 
fragmentary, but sufficient to 
Fie. 51. Chimera phantasma, Jordan and 
Fowler ; opened jaws showing two pairs 
of upper dental plates and one pair of 
mandibular dental plates, nat. size.— 
Existing in Japanese seas. After B. 
Dean. 
show that they represent fishes closely similar to 
those now existing. They are all of extinct 
genera, and most of them belong to species much 
larger than any of modern times; but their den- 
tition corresponds so well with that of the Chimee- 
ridz (Text-fig. 51) that all the Cretaceous forms 
may be referred to this family.’ The dental 
plates are stout, and certain areas, or “ tritors,” 
are specially hardened by the deposition of calca- 
reous salts within and around groups of medullary 
canals, which rise to the functional surface. In 
the lower jaw there is a single pair of such 
plates, meeting at the symphysis. In the upper 
jaw there are two pairs, each meeting closely in 
the middle line. The anterior upper dental 
plates (commonly termed vomerie) are more or 
less cutting or prehensile; while the posterior 
upper dental plates (commonly termed palatine) 
have usually extensive tritoral areas for crushing. 
Among existing species the shape of the dental plates is very variable (‘Text- 
fig. 52), so that in naming the fossils it is well to ignore small differences of 
proportions. 
1 B. Dean, “ Chimeeroid Fishes and their Development,” Carnegie Inst. Publ., no. 32, 1906. 
