4IO S. W. WILLISTON 



brae (Fig. 1 1 , 5, 12) short and flattened, with the upper extremity 

 flattened ovate in section; axis with a broad spine, as in the Polio- 

 sauridae; diapophyses with a continuous articular surface, directed 

 downward and forward, showing holocephalous ribs, as in Ophia- 

 codon. Humerus (Fig. n, i, 10, 11) with the lateral process high 

 up on the shaft; distal extremity much expanded, its plane nearly 

 at a right angle with that of the proximal end; carpus (Fig. 11, 

 3) very much as in Ophiacodon; the first centrale and inner carpalia 

 larger. Pubis (Fig. n, 5) subcircular or subquadrate in outline, 

 with the obturator foramen more or less notched (in the young at 

 least); ischium and proximal tarsal bones (Fig. 11, 2, 6) as in 

 Ophiacodon; femur with a rather slender shaft (Fig. 11,4). Length 

 of animal probably about 5 feet. 



So far as the skeletal bones and teeth are concerned, the genus 

 seems to be related to Ophiacodon, but the pubis shows very distinct- 

 ive characters, and the spines of the vertebrae are relatively much 

 shorter; the humerus is also much more elongated. On the 

 strength of the pubis alone, a very characteristic bone in the Pely- 

 cosauria, the genus is, I think, eliminated from the Poliosauridae 

 or Ophiacodontidae ; and the teeth will exclude it from the Sphena- 

 codontidae. The pubis has a distinct resemblance to one which 

 I figured erroneously as a calcaneum in my American Permian 

 Vertebrates (Plate XXXI, Fig. 10, and Plate XXXII, Fig. 11), 

 from the Craddock bone-bed, Texas; and it is possible that the 

 femur, also figured in this work on Plate XXX, Fig. 4, may belong 

 with this pubis, a yet unnamed genus, that may prove to be related 

 to the present one. 



THE PRIMITIVE STRUCTURE OF THE MANDIBLE IN 

 REPTILES AND AMPHIBIANS 



Until within recent years the mandible of reptiles was thought 

 to be composed of not more than six bones, originally named by 

 Cuvier and Owen the articular, angular, surangular, coronoid, 

 splenial, and dentary. In Sphenodon only was it known that one 

 of these bones, the splenial, was absent. The bone long known 

 as the splenial in the turtles occupies an anomalous position for 

 that bone, overlying the inner side of the articular and remote 



