398 PROCEEDINGS OF UNITED STATES NATIONAL MUSKUM. 



lation. from midpoint of its sides. Gerrhonotus has a liyoidean arcli 

 very similar to the one fouiitl in Opheosaurus. 



In the largest and best specimen that was examined, there were found 

 147 vertebraj with a series of caudal nodules where a tail had been re- 

 placed ; there were 52 [)airs of ribs, and this nund^er was also found in 

 a smaller specimen. The atlas is characteristic of the usual Lacertilian 

 type, and a stumpy odontoid process is found upon the axis. Free ver- 

 tebral ribs are not exhibited until we have passed backwards for three 

 or four segments, but when the series commences it is continuous to 

 within one vertebra of the rudimentary pelvis, and even this interven- 

 ing segment may develop a small free plenrapophysis. These ribs have 

 rounded bodies with laterally compressed and dilated extremities below; 

 the pair when articulated lie in the curve of a broad ellipse that sustains 

 the shape of this lizard's body. They articulate by feebly developed 

 oapitula, at the base of the diapophyses, at the very anterior margin of 

 the centrum of each vertebra, in concave facettes placed there for their 

 accommodation. Commencing with the atlas, the first two or three 

 vertebra? support hypapophyses, that are at first directed downwards, 

 then directly backwards in a sharp point; it is with this segment, too, 

 that the (juadrate neural spine makes its appearance, to be continued 

 throughout the chain, past the pelvis ; to become directed more and 

 more backwards, and more jjointed as we pass through the caudal series. 

 Well-developed pre- and post-zygapophyses are found upon the neural 

 arches of all of the vertebrse, and the cup and ball socket 

 between the centra is ellipsoidal in form and placed 

 transversely on the bone, being concave in front, convex 

 behind. The neural tube, beginning more or 

 less triangular, becomes subelliptical as we 

 pass posteriorly. Caudal vertebme develop 

 sharp, vspine-like diapophyses, that are directed _p. _ 

 outwards at right angles with the neural spines 

 and the chevron bones below, which latter in these segments are in each 

 case a wedge-formed hypapophysis, attached to each vertebra, the trian- 

 gular haemal canal passing through them all. The sternum and scapular 

 archin Opheosaurus is largely cartilagir.ous, though bone tissue is de- 

 posited about the points, where in the higher lizards the glenoid cavity 

 exists, and other localities where additional strength is required. So far 

 as my examinations have extended I have thus far failed to discover the 

 j)resence of a rudimentary pectoral limb ; even the very semblance of 

 the gienoidal socket appears to be missing. The clavicles do not meet 

 in the median line, but their outer extremities artfculate with the ex- 

 panded blade of the scapula on either side, which latter bone is semi- 

 osseous only. A transverse plate, covering the lower borders of the 

 coracoids, is the sole representative of a sternum. The entire apparatus 

 is placed immediately over the trachea, while the outer and exi)anded 

 blades of the scapula? lap over the first and second pleurapophyses. 



