1869.] 265 [Cope. 



These peculiarities have been pointed out by Leidy, who refers the 

 species to the genus Holcodus of Gibbes. Now this genus Leidy 

 shows was nude to include also teeth of Ilyposaurus, and it may be 

 that the name should be restricted to that genus, as its meaning is 

 " grooved tooth," a term not applicable to a Mosasauroid. But as it 

 has been accepted for the Mosasauroid included by Gibbes, by the 

 next writer, Leidy, it must be retained for it, according to the just 

 rule usually followed. There is, however, for us no evidence that the 

 present genus possessed such a tooth; and as the teeth of all the gen- 

 era bear such a close mutual resemblance, I think it must be left for 

 future discovery to determine the application of the genus Holcodus. 



Dorsals transversely ovate, rounded; quadrate bone broad as long, 

 meatus larger. Humerus little contracted medially, with flat shaft; 

 pterygoid teeth eight P. tympanilicus. 



Platecarpus tympaniticus Cope. Holcodus acutidens Leidy, 

 Cretaceous Reptiles N. Am., p. 118, Tab. vii, 4-7; viii, 1-2-7; xi, 

 14; vi.v Gibbes Smithson. Contrib., 1851, H, 7, Tab. i, 3-5 vel Luidy, 

 loc. cit., Tab. x— 17. 



The individual of this Mosasauroid, from which it is known, was 

 of medium size; it was found in the upper cretaceous of Mississippi, 

 near Columbus, by Dr. William Spillman. 



STREPTOSAURIA. 



Under this nanie I have characterized a group of high rank among 

 the Reptiles, which is allied to the Sauropterygia. The diagnosis will 

 be as follows. 



The articular processes of the vertebras, reversed in their directions; 

 viz., the anterior looking downwards, the posterior upwards; the pro- 

 coracoids distinct from the scapula, but confluent with each other 

 and the mcsosternum into a simple breast plate. Mandible with sym- 

 physis. Pelvic arch present; limbs present. Neural arches of ver- 

 tebrae cobssified with centra. 



The characters of this order are altogether peculiar. They are 

 largely derived from an almost complete specimen of Elasmosaurus 

 platyurus Cope in the Museum of the Academy of Natural Sciences of 

 Philadelphia. The vertebral character may be explained on the 

 supposition that the zygosphen and zygantral articulation is present, 

 and the zygapophysial wanting, or that the obliquity of the taces of 

 contact of the zygapophyses is reversed. The genera known are three, 



