in North America of rare Extinct Vertebrates. 55 



{l yellow limestones belonging to the Niobrara group of the 

 Cretaceous Formations "*". 



From these American materials a restoration of the Leiodon 

 modification of the Mosasauroid Lacertians may be under- 

 taken. Of the skull I have little to add to the description 

 given in my paper " On the Rank and Affinities of the 

 Mosasauroids " f. The edentulous production of the pre- 

 maxillary (PI. VIII. fig. 1, a) beyond the alveoli of the 

 — incisors may be noticed as suggestive of a rudiment or 

 beginning of the Ziphioid modification of the cetaceous cra- 

 nium. The mandible is slender, with a low coronoid process 

 and slightly produced angle. A specimen has been found 

 26 inches in length J. 



The vertebral column of Leiodon anceps (PI. VIII. fig. 1) 

 exhibits the range of modification in its several regions 

 characteristic of these great extinct ' Lacertia Natantia'§, 

 and contrasting with the comparative uniformity of the ver- 

 tebras in Python and other Ophidia. 



The atlas (ib. fig. 2) consists, as in lizards||, of a pair of 

 neurapopliyses, n, and a detached hasmapophysis, //,, simula- 

 ting a centrum^}. The transverse exceeds the vertical dia- 

 meter, although the latter is extended by a short obtuse 

 hypapophysial spine, y, less developed than in Mosasaurus**. 

 Each neurapophysis presents a large subconcave facet for 

 articulating with part of the occipital condyle ; a rudimental 

 diapophysis projects from the outer side. 



The axis (ib. fig. 3) consists of a long body, including the 

 proper centrum of the atlas, ca, coalesced with that of the 

 axis, c x. The latter develops a hypapophysis, y, to which 

 is articulated a short hasmapophysis, h. A compressed ver- 

 tical ridge-like process (par-diapophysis, d) extends from each 

 side of the centrum ; it may be for the support of a rudimental 

 cervical rib. 



A few of the succeeding vertebras are characterized by both 

 diapophysis (ib. fig. 4, d) and hypapophysis, y — the latter 

 with a rough articular surface for ligamentous attachment of a 



* Op. cit. p. 54, plate opposite same page, and woodcut p. 57. 



t Quarterly Journal of the Geological Society, vol. xxxiii. p. (j'X>, 

 fig. 15. 



X In the grey shale of the Niobraska chalk ; referred by Prof. Cope to 

 a Leiodon ncjueolicus (op. cit. p. 177). 



§ ' Fossil Reptilia of the Cretaceous Formations,' 4to, 1851, p. 29. 



|| Cuv. Oss. Fosa. 4to, vol. v. pi. vvii. fig. 10. 



^f See 'Annals and Magazine of Natural History,' vol. xx p. 217, 

 figs. 4, 5. 

 ' ** Cuv. <>/'. cit. pi. x.v. i\;x L4, c (after Camper). 



