FISHES OF THE GENUS ACENTROPHORUS. 25 



presence is suggested only by faint radiating grooves crossing 

 the bones of the pala.te in a few specimens. One or two examples 

 show a triangular bone which apparently covered the space in 

 front of the orbit between the nasal and the maxilla. 



The opening of the mouth is very small. It is bounded above 

 by fairly strong paired premaxillse, each supported by a strong 

 process running backwards beneath the nasals to the frontal 

 (text-fig. 5, r.pmx.). The maxillfe are short and delicate ; their 

 slender anterior portion passes inward behind the premaxillse ; 

 posteriorly they ar-e expanded to overlie the lower jaw about the 

 middle of its length. The lower jaw is remarkable chiefly for 

 the fact that in this early genus it has already acquired in the 

 fullest degree the peculiar shape characteristic of the Mesozoic 

 Semionotids and Macrosemiids. As in these later fishes it is 

 very deep behind and shallow in front, with a downward curve 

 in the region of the symphysis. The agreement extends also to 



Text-figure 4. 



cor--. 



sq.-.. 



'-.pa 



Acentrophorus alius. Skull-roof of a specimen witli four parietal plates. In the 

 Kirkby Collection, Hancock Museum. The plates behind the frontal are 

 slightly displaced to the right. About one-and-a-half times natural size. 

 Lettering as in text-fig. 3, with op., operculum. 



the shape of the constituent bones, at any rate to those encasing 

 the outer side of the jaw^ — the angular, coronoid and dentary 

 (text-figs. 1, 5 and 6). An articular ossification was probably 

 present, but though it is sometimes suggested it is not clearly 

 shown. Many specimens show something of the inner side of 

 the jaw, but the splenial was apparently veiy thin, and its boun- 

 daries are lost. The most conspicuous feature exhibited on the 

 inner face of the jaw, as it is actually preserved, is a row of pro- 

 tuberances near the lower margin corresponding to the sensoiy 

 pits in the outer face of the dentary (text-figs. 5 and 13). 



The only mention of teeth in Acentrophorus occurs in Dr. Smith 

 Woodward's Catalogue (1895, pp. 51 and 52). Traquair states 

 that he found none, and Kirkby's account makes no mention of 

 any. It is evident, however, that Kirkby had discovered them 

 subsequently, for the fine specimen represented in text-fig. 5 bears 



