THE PHYTOSAURIA OF THE TRIAS 153 



features well. It is a dorsal vertebra, one of the most anterior. 

 It corresponds in a general way with those of the same region in 

 Rhytiodin carolinensis Emmons and may therefore be compared 

 with one of the anterior thoracics of that form figured by McGregor.' 

 The vertebra of Angistorhinus gracilis is considerably the larger and 

 the proportions are somewhat different. The centrum is very 

 much constricted laterally, measuring but 29 mm. in the middle. 

 The posterior face of the centrum is approximately circular, meas- 

 uring about 54 mm. in diameter. The transverse diameter of the 

 anterior face is somewhat greater than the vertical diameter, due 

 to the enlarged articulation for the tuberosity of the rib. These 

 diameters measure 68 mm. and 69 mm. respectively. The dia- 

 pophyses form heavy transverse processes extending horizontally 

 at the level of the neural canal and expanding distally. They are 

 approximately triangular in section with one vertex directed down- 

 ward. The centrum is considerably excavated below the base of 

 the diapophyses, leaving them supported for a short distance 

 laterally by two thin buttresses which are confluent below with the 

 anterior and posterior rims of the centrum. The capitular articu- 

 lation (if it be the true articular surface) is slightly concave. The 

 articulation for the tuberosity is low down on the anterior border 

 of the centrum, is convex, and as large as the capitular articulation. 

 The posterior zygopophyses are missing. The anterior zygopophy- 

 ses have a considerable anterior prominence. They are large, are 

 directed up and inward at an angle of about 45° from the horizontal, 

 and, though this measurement may be exaggerated by distortion, 

 measure 44 mm. between the center of the articular faces. The 

 spine is broken off about 31 mm. above the neural canal. The 

 total height of the vertebra must have been at least 150 mm., how- 

 ever, and probably more. The following table of measurements 

 will serve to show some of the differences in the two forms: 



Rhylidodon carolinensis Angistorhinus gracilis 



Length of centrum 38 mm 45 mm. 



Height of centrum 42 mm 54 mm. 



Total height 120 mm (at least) 150 mm. 



Width across diapophyses. . . 104 mm 156 mm. 



' Op. cit., p. 66, Figs. 15, 15a, and PL 8, Figs. 10, loa. 



