ORDER GANOIDEI. 



975 



the adult, the presence of five rows of scutes formed of true bone 

 on the body, and the sculptured cranial bones. The representatives 

 of the existing genera Acipe?iser (fig. 911) and Scaphirhynchus are 

 found in many of the larger rivers of the Northern Hemisphere. 

 Scutes referred to Acipenser occur in the London Clay, and in some 

 higher Tertiary beds ; while spines from the English Upper Eocene 



91T. — The Sturgeon (Acipenser sturio). Much reduced. 



and the Pliocene of Montpellier probably also belong to the same 

 genus. A single scute has also been described from the Miocene 

 of Virginia under the same name. 



Family Polyodontid;E. — In this family, now represented by 

 Polyodon of the Mississippi and Psephnrits of two rivers in China, the 

 skin is typically nearly or quite naked, the mouth is of enormous 

 width, and the jaws carry minute teeth. In a fossil state this family 

 is represented by Crossopholis, from the Eocene of Wyoming, which 

 displays many points of resemblance to Polyodon, but is remarkable 

 for the possession of small pectinated scales, which are not confined 

 to the upper lobe of the tail. The cranial bones are of the type of 

 those of Polyodon, but the shorter rostrum indicates a resemblance to 

 Psephurus. The scales are numerous, and are arranged in oblique 

 rows, which are not quite in con- 

 tact with one another. The reten- 

 tion of the scales in this genus in- 

 dicates that we have to do with a 

 much less specialised member of 

 the family than the existing forms. 

 The genus Macropetalichthys, from 

 the Devonian of both North Amer- 

 ica and the Eifel, has frequently 

 been referred to the Polyodo7itidce, 

 but without any sufficient evidence, 

 and its serial position must for the 

 present remain undetermined. The 

 cranium (fig. 912) is short and 



broad, with the orbits completely surrounded by bone, and the 

 middle line occupied anteriorly by a diamond-shaped ethmoidal 

 shield, which articulates posteriorly with a process from the squared 

 hinder shield. 



912 



Diagram of the frontal aspect 

 of the cranium of M acropetalichthys Sidli- 

 vani ; from the Devonian of North Amer- 

 ica. Much reduced. (After Newberry.) 



