DESCKIPTION AND COMPARISONS 399 



was regarded as more or less provisional at the time, for the reason that 

 the cranial osteology, as far as it had then been worked out, showed 

 anomalous features. It was observed that while the dentition, outline 

 of head, and majority of body characters indicated a position with the 

 Pholidophoridas, the arrangement of facial bones was suggestive oi Eugna- 

 iJiidse. Inasmuch, however, as the sutures between the thin cheek-plates 

 were extremely difficult to determine and their homologies therefore 

 more or less uncertain, it seemed wisest to rely upon body characters 

 for deciding the generic affinities. 



Comparison of Family Characters 



The i^roblem which then arose has not yet been solved by the acqui- 

 sition of more perfect material ; Mr Darton, in fact, fears that the fish- 

 bearing stratum at the original localit}^ is exhausted, and he has not been 

 successful in discovering fresh outcrops. A word or two may be offered 

 to explain the exact nature of the difficult}^ Such forms as EugnatJms, 

 Heterolepidotus, Cnturus, and the like are characterized by the possession 

 of two very large postorbitals in advance of a narrow preoperculum, a 

 ring of small circumorbitals, and at least one large preorbital plate. All 

 these bones are moderately robust and externally enameled ; in most 

 cases they are also more or less ornamented. In the Pholidophoridse. on 

 the other hand, the external bones are very delicate, the [)Ostorbitals of 

 relatively small size, and together with the circumorbitals complete!}^ 

 cover the cheek. The preoperculum is a large and mesially broad tri- 

 angular plate ; both the inter- and suboperculum are relatively larger 

 than in Eagnathidss, the latter plate being separated from the operculum 

 by an oblique suture. Other differences between the two families are 

 set forth in the following table, the characters being compiled from A. S. 

 AVoodward's diagnoses : * 



Eugnatlms, Heterolepidotus, etcetera Pholidophorus 



Cranial and facial bones moderately External bones delicate, smooth or 



robust, usually ornamented with tuber- very feebly ornamented, preoperculum 



culations or rugae ; two very large post- triangular, broad mesially ; interopercu- 



(or sub-) orbitals in advance of a narrow, lum and suboperculum large, the latter 



elongate preoperculum ; interoperculum divided from the trapezoidal operculum 



small. by an oblique suture. 



Mandible complex, consisting of den- Mandible simple, consisting of but 



tary, splenial, coronoid, and angular ele- two elements (coronoid and splenial not 



ments ; teeth large, in more than one observed); teeth minute and conical; 



series ; a large gular plate present. no gular plates. 



♦ Catalogue Fossil Fishes British Museiiin, part iii, 1895. 



