GEOLOGICAL SURVEY OF THE TEREITORIES. 341 



Dr. Leidy applied the name of XipJiactinus to a genus indicated by a 

 spine in some degrees like those regarded above as pectorals of Sauro- 

 dontidw. It is quite distinct from those assigned to Fortheus and IcJi- 

 thyodecteSj and may belong to SaurocephaluSj as already suggested, or to 

 another genus. 



POETHEUS, Cope. 



Proc. Amer. PMlos. Soc, 1871, p. 173 ; he. cit, 1872, February. 



Teeth subcylindric, without serrate or cutting edges, occupying the pre- 

 maxillary, maxillary, and dentary bones. Sizes irregular; the premax- 

 illary, medium maxillary, and anterior dentary teeth much enlarged. 

 No foramina on inner face of jaws. Teeth on the premaxillary reduced 

 in number. Opercular and preopercular bones very thin. Cranial bones 

 not sculptured. 



The fishes of this genus were rapacious, and, so far as known, of large 

 size. They constitute the most formidable type of physostomous fishes 

 known. Three species are known to the writer, one from teeth only, 

 from the Miocene of North Carolina, but not certainly known not to be 

 an intrusive Cretaceous fossil, and two from Kansas. The latter are 

 represented by more or less numerous fragments of eleven individuals, 

 three of which possess large portions of the cranium, one almost entirely 

 complete. Two of the remainder embrace jaws, and one, a large part 

 of the vertebral column, with segmented rays. In one, these rays were 

 found with the cutting, compound ray above described, while the simple, 

 flat, pectoral rays occur with several specimens. In none have any 

 traces of symmetrical spinous rays been found, nor strong interneurals 

 capable of supporting such. In none of the more perfect specimens 

 with crania have the segmented rays been found, but the fossil of 

 P. thaumaSy where they occur, is represented by a vertebral column and 

 its appendages, which do not differ appreciably from those of P. molossus. 



In the cranium of this genus there is a well-marked supraorbital rim. 

 Each opisthotic forms a prominent angle directed posteriorly on each 

 side of the exoccipital. The parasphenoid is a stout and narrow bone, 

 deeply emargioate behind for the i)assage of the muscular canal. It 

 has a transverse expansion in front of the base of the prootic, which 

 rests on a backward continuation of the same. This expansion is 

 pierced behind by two round foramina. The shaft is abruptly contracted 

 in front of the expansion and is trigou al in section. The prefrontal extends 

 downward and forward and carries inferior and anterior articular faces, 

 the latter vertically transverse. The x)ostero-inferior i^ortion of the 

 ethmoid bears on its posterior extremity a concave articular face, which 

 oijposes that of the prefrontal. The floor of the brain-case in front is 

 supported by a vertical style, which is bifurcate above and rests on the 

 parasphenoid. 



Of the teeth in general, it may be added that their pulp cavity is 

 rather large at the base but rapidly diminishes in the crown. The 

 mode of succession is by direct displacement li'om below. The young 

 crown rose into the pulp cavity and destroyed the vitality of the crown 

 while the root was absorbed. i«Iumerous empty alveoli are to be found in 

 all the jaws of this genus, in which examination will often detect the 

 apex of the crown of the young tooth. 



The vertebra) in this genus are rather short, but not so nuich so as in 

 sharks. In P. thmmias nearly eighty dorsals a'nd caudals were preserved; 

 those without lateral giooves or cervicals (the luime not appropriate) 



