398 PROCEEDINGS OF UNITED STATES NATIONAL MUSHUM. 



lation, from midpoint of its sides. GerrJionotus has a liyoidean arch 

 very similar to the one found 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 pairs of ribs, and this number 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 pleurapophysis. 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 

 capitula, 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 

 vertebrne. support hypapophyses, that are at first directed downwards, 

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

 that the quadrate neural spine makes its appearance, to be continued 

 throughout the chain, past the i)elvis ; to become directed more and 

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

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

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

 between the ceutra 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 i^osteriorly. Caudal vertebrae develoj) 

 sharp, spine-like diapophyses, that are directed ^ / „ 

 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 

 arch in OpJieosaurus is largely cartilaginous, 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 

 presence of a rudimentary i^ectoral limb ; even the very semblance of 

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

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

 l^anded 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 expanded 

 blades of the scapulte lap over the first and second pleurapophyses. » 



