EASTMAN: FISHES FROM UPPER EOCENE OF MONTE BOLCA. 327 



Order Berycomorphi. 

 The view is commonly entertained that the fishes belonging to this order are 

 very generalized, as is shown by such characters as the large number of pelvic 

 fin-rays, and the persistence of the pneumatic duct in certain genera, as well as the 

 widespread distribution and importance of the group so long ago as the Cretaceous. 

 The family Berycidse is represented in the Upper Eocene fauna of Monte Bolca by 

 two genera, Holocentrum and Myripristis, remains of which are not uncommon. 

 The typical species of these genera, which are represented in the collections of the 

 Museum, have already been considered in the Catalog of Fossil Fishes published 

 in Volume IV of the Memoirs of this institution. 



Order Heterosomata. 



Physoclistic Teleosts with asymmetrical craniimi and strongly compressed 

 body, the precaudal region short; pelvic bones directly attached to the cleithra 

 (clavicles) ; fins without spines. 



The Heterosomata, or Flat-fishes, are to be regarded as aberrant, strongly 

 compressed Perciformes or a derivative from that stock, instead of being asym- 

 metrical Gadoids, as was formerly svipposed. They differ from all other fishes in 

 having an asymmetrical cranium; both eyes are on one side in the adult, this side 

 being uppermost and pigmented, whilst the lower or eyeless side is usually devoid 

 of pigment. 



Boulenger has expressed the view that the Upper Eocene genus Amphistiuni 

 is allied to the symmetrical ancestor of the flat-fishes, and this opinion is also shared 

 by Regan, who, however, regards Amphistium as a Percoid not far removed from 

 Platax, and approaching in some respects to the existing Psettus. As true Soles 

 accompany Amphistiuni in the Upper Eocene, the ancestral form from which flat- 

 fishes are derived must have been evolved at a still earlier period. Valuable de- 

 scriptions and figures of the crania in flat-fishes were published by Traquair^ in 

 1865, and various classificatory schemes have been proposed for the group by recent 



writers. ** 



Suborder Pleuronectoidea. 



Dorsal fin extending forward on the head at least to above the eye ; all the fin- 

 rays articulated, each pelvic fin of six or fewer rays. No supramaxillary bone; 

 no palatine teeth; lower edge of urohyal deeply emarginate, so that the bone appears 



8 Trans. Linn. Soc, Vol. XXV, 1865, pp. 263-296. 



» Kyle, H. M., Kept. Fisheries Board Scotland, Vol. XVIII, 1900, pp. 335-368.— Boulenger, G. A., 

 Cambridge Natural History, Fishes, 1904.— Regan, C. T., Origin and Evolution of the Teleostean Fishes 

 of the order Heterosomata. Ann. Mag. Nat. Hist., (8), Vol. VI, 1910, pp. 484-496. 



