1885.] ^31 [Cope. 



that of the others. As its ampullar orifice is also the largest of all, I 

 suppose this increased diameter to be partly normal ; but it may be partly 

 abnormal, as its walls are irregular and rough. 



The fenestra ovalis is not preserved in this specimen, but can be seen in 

 the crania of the species Diadectes jjhaseolinus and Empedias molaris above 

 mentioned.* The vestibule or a diverticulum from it is produced upwards 

 and backwards, and terminates in a round os. This is clearly not a tym- 

 panic chamber, nor is it a rudimental cochlea. It does not appear to be 

 homologous with the recessus labyrinthi, since that cavity is not perforated 

 by the fenestra ovalis. It appears to be a prolongation outwards of the 

 vestibule and sacculus, which may be observed in a less degree in the 

 genus Edaphosaurus (Cope), also from the Texas Permian formation. 

 Here the adjacent bones are produced slightly outwards, and the fenestra 

 ovalis is closed by a large stapes similar in external form to the one I have 

 described in the Clepsydrops leptocephalus.\ Its more intimate structure I 

 have not yet examined. % 



The result of this examination into the structure of the auditory organs 

 in the Diadectidse may be stated as follows : The semicircular canals have 

 the structure common to all Gnathostomatous Chordata. The internal 

 wall of the vestibule remains unossified as in many fishes and a few 

 batrachians. There is no rudiment of the cochlea, but the vestibule is pro- 

 duced outwards and upwards to the fenestra ovalis, in a way unknown in 

 any other family of vertebrates. 



I may add that, in the specimen examined, the semicircular canals were 

 filled with a white calcareous powder, probably derived from the commi- 

 nution of otolites. 



EXPLANATION OF PLATE. 



Figs. 1, 2 and 3 cast of cranial cavity, natural size. As the basicranial 

 axis is lost, the inferior outline posteriorly is provisional only. 



Fig. 1, from above. 



Fig. 2, from the left side. 



Fig. 3, from behind. 



The letters signify as follows: m., medulla; cb., cerebellum; opL, 

 optic lobe ; ep., epiphysis ; ppe., posterior process of epiphysis ; If., lateral 

 foramen; h., region of cerebral hemispheres ; v., cast of vestibule; hap., 



*See skull of E. molaris, Proceedings Amer. Philosoph. Society, 1881, Plate v, 

 figs, a and b, where the fenestra is represented. 



fSee Proceedings Amer. Philosoph. Society, 1884, p. 41. 



% Professor Owen has figured (Todd's Encyclopedia, art. Monotremata) a struc- 

 ture in Echidna, which looks remarkably like that here described. This is a 

 tubular elongation of the meatus auditorius externus with more or less carti- 

 laginous walls. This structure might be regarded as homologous with that dis- 

 played by the Empedias, could we imagine that with their diminution in size 

 in tbeMonolreme, theossicula auditus had retreated within this tube preceding 

 the membranum tympani, from a position at its distal, to one at its proximal 

 extremity. But such a supposition has as yet no foundation, and the very 

 similar parts in the two types may have no homology. 



