28 AMPHIBIA AND PISCES OF THE PERMIAN OF NORTH AMERICA 



"The absence of centrum and presence of neural spine and articular 

 faces of the neuropophyses, with the well-developed diapophyses, distin- 

 guish this genus from Irimerorhachis. The large intercentra and articular 

 faces of the neural arch distinguish it from Archegosaurus." 



Char. Specif.: "The Rhachitomous valens is a much larger species than 

 the Trimerorhachis insignis, equaling or exceeding the Empedocles alatus. 

 The intercentra are very robust; the posterior face is nearly straight, while 

 the inferior border of the anterior face curves backward to meet the former 

 at an angle. The inferior face is convex transversely and slightly concave 

 anteroposteriorly. The tubercular rib facets are oval, and are narrowed 

 downwards and forwards. The side of the neurapophysis describes a curve 

 which rises a little to the superior part of the extremity of the diapophysis. 

 The zygapophysial surfaces are as wide as long, and a little oblique. The 

 neural spine is not very elevated and is very robust; its section is a longi- 

 tudinal oval. The summit is truncated and thickened laterally. 



"Measurements. 



"Diameter of intercentrum : m 



Transverse 0.035 



Antero-posterior 023 



Expanse of the diapophyses 073 



Length of tubercular surface of do 022 



Elevation of neural arch 071 



Elevation of neural spine 040 



Antero-posterior diameter of summit of do 044" 



The mistakes in these descriptions and the identity of the specimens with 

 Eryops megacephalus are now sufficiently obvious, but they are reprinted 

 to show the development of our ideas concerning these forms. 



Revised description: 



1. Large, 2 to 2.5 meters. 



2. Otic notch present. 



3. Orbits proportionately small, a little posterior to the middle of 



the skull. 



4. Nares some distance from the anterior and lateral edges. 



5. Angle of lower jaw not produced behind the quadrate. 



6. Lower jaw rather wide, vertically. 



7. Skull covered with a coarse sculpture, especially strong on pos- 



terior portion. 



8. Teeth irregular in size. Large tusks, in pairs, on prevomers, 



palatines, and maxillaries. 



9. Dentine simply but strongly infolded. 



ID. Fine teeth on prevomers, pterygoids, and parasphenoid. 



11. Ribs expanded with angular extension of posterior edge, 



12. A single sacral rib. 



13. Pelvis narrow, with a long and narrow symphysis. 



14. Spines of dorsal vertebrae rugose at apex. 



15. Spines of caudal vertebrae with bifurcate apices. 



16. Humerus short, with powerful processes; proximal and distal 



extremities at right angles. 



17. Femur with a narrow and high ridge on posterior face. 



18. Feet short and wide. 



