A DESCRIPTION OF THE GENUS DIMETRODON, COPE. 19 



present. The centra are strongly compressed, and on the anterior part of the column 

 have an obtuse hypophysial keel. The intercentra display equal width on the inferior 

 surface, and are abruptly rounded at the extremities. The last one preserved is between 

 the second and third caudal centra. The sacrum is rather robust. The sacral vertebrae 

 are free, have well-developed neural spines and large free ribs for the ilium. 



A new species of Edaphosaurus, E. microdus, is described. The genus Edapho- 

 saurus was established on E. pogonias, represented by a specimen, which included 

 only a distorted cranium. The new species is based on numerous vertebra? and ribs and 

 the dentigerous plates of both jaws. The vertebras possess enormously elongated neural 

 spines as in Dimetrodon. The centra have a facet on the anterior edge above the middle 

 for the head of the rib, as in a mammal. It is not repeated on the posterior edge of any 

 of the thirteen centra preserved. The ribs are only compressed proximally. Distally 

 their section is a wide oval. 



The centra are rather elongate, and the foramen chordm dorsalis is rather large. No 

 intercentra are preserved, and if present they must have been very small, as the inferior 

 rim of the centrum is not beveled to receive one. The neural spines have transverse 

 processes which commence near the base, and project at intervals from the sides. 



A special portion of the paper treats the posterior foot in the Pelycosauria (Clepsy- 

 drops natalis, PI. I, Figs. 5, 6). 



The astragalus and calcaneum are large and well-specialized bones free from each 

 other and the other tarsal elements. The navicular is distinct. There are four tarsals 

 in the distal series. Three are articulated with the metatarsals 1 to 3, the fourth 

 with metatarsal 4 and 5. These elements are tarsal 1 (entocuneiform), tarsal 2 (meso- 

 cuneiform), tarsal 3 (ectocuneiform), and tarsal 4 and 5 (cuboid). There is a face on the 

 astragalus for another element, which Cope thinks might have been a spur, as in the 

 Monotremata. 



The following conclusions are reached : 



"1. The relations and number of the bones of the posterior foot are those of the 

 Mammalia much more than those of the Reptilia. 2. The relations of the astragalus and 

 calcaneum to each other are as in the Monotreme Platypus anatinas. 3. The articulation 

 of the fibula with both calcaneum and astragalus is as in the Monotreme order of mam- 

 mals. 4. The separate articulation of the anterior part of the astragalus with the tibia is 

 as in the same order. 5. The presence of a facet for the articulation of a spur is as in the 

 same order. 6. The posterior-exterior direction of the digits is as in the known species 

 of Monotremeta." 



" Thus the characters of the posterior foot of the Pelycosauria confirm the evidences 

 of Monotreme affinity observed by Prof. Owen and myself in the bones of the legs, 



