SYSTEMATIC REVISION 4I 



or axis. This is similar to the structure already observed in Eryops and, 

 the parts being in place, should explain those of that genus. The portion 

 of the atlas which represents the intercentrum is divided into two lateral 

 portions, each of which has the form of an entire intercentrum, i. e., cres- 

 ceniic. The intercentrum of a cervical of a large species of this group is 

 wider than that of the other vertebrae and presents two articular facets 

 anteriorly." 



In the "American Naturalist" he adds: 



"This genus presents the most imperfect vertebrae known in the order 

 (Rhachitonii). It differs from all others, including Archegosaurus, in the 

 lack of a distinct neural spine. Its humeri do not. display condyles, but had 

 cartilaginous articular surfaces. The teeth are rather small and of equal 

 size, except a large one or two inside the external series near the anterior 

 part of the mouth." 



Revised description: 



1. Small, not exceeding 500 centimeters in length. 



2. Skull elongate, flat; orbits small, in anterior half of skull; with- 



out elevated rims; no interorbital depression. 



3. Nares not terminal. 



4. No preorbital depressions. 



5. Sculpture finely reticulate; edges of skull smooth. 



6. Tabulare not prolonged into points. Posterior edge of skull 



not deeply concave. 



7. Occipital condyle not divided. 



8. A double row of teeth on maxillary. Teeth not enlarged, except 



on anterior end of mandible, and palatine tusks (?). 



9. Intercentra thin, not constricting the notochord. 



Trimerorhachis insignis Cope. 



Cope, Proc. Am. Phil. Soc, vol. xvii, 1878, p. 524; Proc. Am. Phil. Soc, vol. xix, 



1880, p. 54; Proc. Am. Phil. Soc, vol. xx, 1883, p. 630. 

 Broili, Paleontographica, Bd. li, 1904, p. 39. 



Type and paratype: The same as the genus. 



Original description: "There are two large tusks at the anterior extremity 

 of the inner superior row of teeth, and two similar ones on the plate-like 

 element above described (see description of the genus). The inferior border 

 of the mandible rises gradually posteriorly to below the posterior border of 

 the glenoid cavity, behind which it is a short vertical and compressed angu- 

 lar process, which is rounded in profile. There is a patch of small teeth inside 

 of the posterior extremity of the mandibular series. The mandible closes 

 inside of the posterior part of the quadratojugal arch. There is a groove 

 near the inferior margin of the inferior face of the mandible; external to 

 this the surface is marked with elongate, shallow pits. The sculpture of 

 the external side of the ramus is less pronounced, and the pits are smallest 

 near the angle. The pits of the top of the cranium are coarse and well 

 defined. The fragment of maxillary bone is broken off four teeth behind the 

 tusks, and the neural opening has contracted but little at that point. The 

 sculpture of the anterior portion of the maxillary is coarsely reticulate. 



"The diapophyses of the centrum are oblique rhomboids in form, the 

 anterior upper side receiving the neural arch. The external surface is con- 

 cave and smooth. The median element, which I call the intercentrum, is a 



