182 SATJROPTERYGIA. 



general structure with those of the type species. Polycotylus of 

 the American Cretaceous is characterized by the deeply cupped 

 terminal faces of the vertebral centra, and probably corresponds, 

 at least in part, to the undermentioned Ccelospondyline group ; 

 for which it might apparently be adopted if it be considered 

 advisable to raise that group to generic rank. Piptomerus from the 

 same deposits is stated to be distinguished from Polycotylus by the 

 absence of anchylosis of the arches and cervical ribs with the 

 centra, but it has yet to be shown that this feature is not due to 

 immaturity. Oligosomus is based on a caudal vertebra from the 

 Cretaceous of "Wyoming. Whether the forms described by Cope 

 under the names of Uronautes 1 and Oroplwsaurus 2 belong to the 

 present genus in the sense in which it is employed here cannot yet 

 be determined. The names Murcenosaurus and Colymbosaurus are 

 noticed in the sequel. Trinacroiaerum from the Cretaceous of 

 Kansas, which is not yet figured, has three bones articulating with 

 the propodials, and the neck is said to be short. Although in the 

 Ichthyopterygia the number of bones articulating with the humerus 

 has been taken as a generic character, yet it seems unadvisable to 

 follow the same course in the present order, since this would lead 

 to making a number of indefinable genera, and would involve 

 the separation of such apparently closely allied forms as 0. tro- 

 clianterius and C. eurymerus. The species of the two groups are 

 arranged in the order of their sequence in time. 



A. Ccelospondyline Group. 



Vertebrae with the centra short or moderately elongated, and 

 their terminal faces more or less deeply cupped ; cervicals with the 

 neural spines short, and the zygosphenes and zygapophyses less 

 complex than in the next group ; usually no anchylosis of the 

 chevrons with the caudal centra. Humerus relatively large, frequently 

 much expanded distally, with the trochanter very large and ex- 

 tending obliquely outwards from the shaft ; when only radius and 

 ulna articulate with the humerus, the ulna often greatly elongated 

 in the transverse direction. 



Probably equivalent, at least in part, to Polycotylus 3 , Cope ; and 

 apparently including Colymbosaurus, Seeley 4 . 



1 Proc. Ac. Nat. Sci. Philad. for 1876, p. 345 (1876). 



2 Original reference not found ; occurs in Amer. Nat. vol. xxi. p. 565 (1887). 



3 In the description in the Amer. Nat. vol. xxi. p. 564, the term 'projpodials 

 should be metapoclials. 



4 Both the detached pectoral girdle (fig. 59) and the skeleton of Plesiosaurus 

 mec/adirus were grouped together in the description of this genus, so that it is 

 difficult to say which is the type (see p. 191). 



