CROCODILIANS, LIZARDS, AND SNAKES. 507 



wanting. Sphenoid and basioccipital separated by suture; descending 

 tuberosities of the latter strong, compressed. Epipterygoid originating 

 below opposite basii)terygoid ; above resting on anterior process of 

 petrosal, and toucliiug parietal Just behind au obtuse descending angle 

 of the decurved border of the same. Subforaminal portion of petrosal 

 shorter than supraforaminal portion, inclosing a very narrow down- 

 looking groove. Basioccipital and exoccipitals coossified; condyle 

 small, simple. 



Meckel's canal closed except distally, where it is open on the under 

 side of the ramus. Corouoid developed anteriorly on external face of 

 ramus, the dentary not reaching behind its anterior border. Splenial 

 elongate, partly external; angular mostly external; surangular con- 

 fluent w ith articular. Angle horizontal, incurved, with rounded border 

 and concave superior surface. 



Hyoid apparatus displays no second and rather short first ceratobran- 

 chials. Hypohyal rather long, the ceratohyal extending a little beyond 

 its extremity, and widened at the posterior third. A free epibranchial, 

 which has a bifurcate anterior extremity at that of the ceratohyal, and 

 extends posteriorly but little behind the ceratobranchial. 



Five cervical intercentra, and two cervicals besides atlas without ribs. 

 The odontoid is coossitied with the axis. Eibs extend to sacrum. 

 Sacral diapophyses distinct from each other. Dorsal vertebra? without 

 zygosphen, prezygapophyseal facets not continued on neural arch. 

 Caudal diapophyses present; centraof middle region segmented through 

 tliem ; chevron bones intercentral. Neural spines low, higher on caudal 

 region. 



Suprascapula much larger than scapula; no proscapula. One large 

 coracoid notch. Sternum without fontanelle, with three ribs and two 

 attached to xiplioid rod. 



Ilium without prominent angulus crisUe; acetabulum entire; j)ubes 

 uniting at an acute angle; pectineal angle medium. Ischia with promi- 

 nent tuber. 



The detailed common characters of this genus will be found more 

 fully among the si^ecific description. The chief peculiarity is in the fold 

 or strip of soft skin on each side, usually covered by the overlapping of 

 the lower sheet on the upper. The dorsal scales are arranged in trans- 

 verse series on each side, which pass a little obliquely backward on 

 the back, so as to meet at an obtuse angle and not always evenly. The 

 Scales of the tail are in thick bony rings, easily separating, so that it is 

 very rare to see a specimen which has not lost its tail and had it repro- 

 duced. In this case the indications of length afforded by the stump 

 are very uncertain, and have given rise to grave errors in framing spe- 

 cific characters. It is probable that in none is it less than one and one- 

 half or one and three-fourth times the head and body. 



In the North American species there are always twelve longitudinal 

 rows of ventral scales, and fourteen to sixteen dorsal; the latter usually 



