BEDS OK THE PERMIAN EPOCH IX NORTH AMEUICA. 293 



Clavicles. — The clavicles in the genus Dimeti-odon are well developed elements. 

 They consist of a vertical narrow and a horizontal expanded portion. The anterior 

 border of the bone is rounded ; the internal border is serrate or semidigitate. If the 

 latter unites -with the epistenunn l)y suture it must be by a very open one. This por- 

 tion is more expanded than in the monotreme mammalia; while the episternum is 

 more produced posteriorly. Tlie tyjic of structure of this [lart of the scapular arch is 

 less mammalian than is liiat of Empedias above referred to, but is not far removed 

 from the latter. 



Posterior foot. — A posterior foot of a species of Pelycosaurian, from Xe Mexico, 

 displays the characters more perfectly than any specimen in my collection. It con- 

 firms the inferences I have derived from the posterior foot of Clepsydrops natalis Cope, 

 as to the mammalian affiliations of this order of reptiles. Thus it has the same number 

 of tarsal bones distributed in the same manner. That is an astragalus and a calca- 

 neum in the proximal series ; then a navicular distad to the astragalus, which is 

 succeeded by three cuneiform bones. Distad to the calcaneum is but a single bone, 

 the cuboid. The specimen described differs in some important particulai's from tliat 

 of the species above mentioned. Thus the astragalus and navicular together areas 

 long as the calcaneum, while in the C. natalis, the calcaneum and astragalus have 

 equal lengths. It is possible that there are but four digits in the posterior foot ; at 

 least I can only find one metatarsal in connection with the cuboid. The internal edge 

 of the astragalus is broken away, so that the presence of the spur or a homologous 

 digit caimot be substantiated. 



As the asti'agalus of Dimetrodon is closely similar to that of Clepsydrops, the 

 species now described does not enter that genus. The rather numerous vertebi'te 

 which accompany it reselnble, on the other hand, those of Theropleura, and it may be 

 that they belong to a species of that genus. By comparison of the i)late, with the cut 

 given under the head oi Enjops plati/jrus, the dill'erence between that type and this 

 may be perceived. 



The lettering of the ligure is as follows: ^s., Astragalus ; 6\<., Calcaneum ; Cn., 

 Cuboid ; Wa., Xavicular ; Mic, Entoeuneiform ; Msc, Mesocuneiform ; JEcc, Ecto- 

 cuneiform ; i-iv. Metatarsals. 



N.\OS.VURUS CLAVIGKU Cope. American Xiitumli.sl, June, 188G. 



Char. gen. — Xaosaurus differs from Dimetrodon only in the presence of transverse 

 l)rocesses on the neural spines. 



The above named very peculiar si)ecies is congeneric with the Saurian described 

 under the name of Dimetrodon cruciger Cope. The neural spines are not quite ho ele- 



