BEOS OF THE PEliMIAN Kl'OCU IN NOUTU AMERICA. 297 



the distill pait of the astragahis has, however, not yet been discovered, and this bone 

 may be that missing piece. Some probability attaches to this identification from the 

 fact that the corresi)onding element in Ornithorhynchus (PI. Ill, fig. 8), is burred in 

 the same manner at its proximal extremity. The present piece is, however, longer 

 than the bone of Ornithorhynchus, and has much more the usual character of a meta- 

 tarsal. It is not jjcrforate at the base, and has only the usual medullary cavity. It 

 is flat on one side, and convex on the other. It must, however, be regarded as proba- 

 ble that fi'om a more or less normal metatarsal, the basal bone of the spur of Ornitho- 

 rhynchus has been derived ; the spur proper representing one or more phalanges. 



That the posterior foot of Vertebrata includes six toes is maintained by Baur. 

 This is confirmed by the presence of a digit within the hallux in various Batrachia 

 Anura. In Rana catesheiana this digit has three segments, a metatarsal and two 

 phalanges, the former resting directly on the astragalus. This digit appears to have 

 been present in the ClepsydropidiE. 



EXPLANATION OF PLATES. 



PLATE II. 



Figs. 1-2. Niiosaurus clntiger Cope. From the Permian formation of Texas. 



Fig. 1. Posterior part of skull, left side ; one-half natural size. Restored from skulls of N. crticiger and Dimelrodoii 

 incmtm. Pa, Parietal bone ; J, jugal ; QJ. Quadralojugal ; Opo, Opisthotic ; Sq. Squamosal ; Q, Quadrate ; Pt, Ptery- 

 goid. 



Fig. 2. Part of palatopterygoid arch of the same from below ; three-quarters natural size. 



Fig. 3. Naosaurus mierodus, part of palatopterygoid arch from below ; three-quarters natural size. 



Figs. 4-5. Ectocynodon incisirms, skull ; from the Permian formation of Texas ; natural size. 



Fig. 4. From above. 



Fig. 5. From bclow. 



PLATE in. 



Figs. 1-2. Naosaurus claviger, dorsal vertebras ; one-half natural size. 



Fig. 1. Front view. 



Fig. 2. Right side. 



Fig. 3. Naosaurus cruciger, neural arch and spine, from front ; one-half natural size ; centrum in outline. 



Fig. 4. Naosaurus mierodus, distal view of distal end of ndiiral spine ; three-fourths natural size. 



Fig. .5. Dimtlrodon? sp., sternum, inferior face. 



Fig. (i. Theropleura sp. ?, pari of posterior foot ; three-fourths natural size. As, Astragalus ; Ga, Calcancum ; Na, 

 Navicular ; C«, Cuboid ; Ece, Ectocuueiform ; .)/, Mesocuneiform ; Enc, Entocuneiforra ; phalanges partly restored. 



Fig. 6a. Calcancum, anterior view. 



Fig. 7. Supposed first metatarsal of Pelycosaurian, twice natural size ; 7 and la, opposite lateral views , b, 

 proximal end. 



Fig. 8. Ornithorhynchus anatinus, internal digit or spur, from Owen, Anatomy of Vertebrata, ii, flg. 199 ; a, meta- 

 tarsal ; J, phalange : c, metatarsal, proximal view. 



