THE OSTEOLOGY OF THE DIADECTIDAE 137 



representatives of the membranous ampulhe at the junction of two pairs of semi- 

 circular canals. 



On sawing open the periotic bones, which here form a continuous mass, the 

 following is seen to be the direction of the semicircular canals: The superior 

 canal is horizontal. The second canal from the posterior ampulla descends 

 forward, and, after a course a little longer than that of the horizontal canal, turns 

 posteriorly. The inferior canal from the anterior ampulla also descends, and, 

 after a shorter course than the canal last mentioned, also turns backward and 

 joins it, the two forming a single canal, which enters the vestibule by the single 

 posterior foramen already described. The lumen of the longer perpendicular 

 canal is much larger than 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 Diadectus phaseolinus and Empedias molaris above mentioned. 

 The vestibule, or a diverticulum from it, is produced upward and backward, 

 and terminates in a round os. This is clearly not a tympanic 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 promulgation outward 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 outward, 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 Diadectidae may be stated as follows: The semicircular canals have the 

 structure common to all the 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 produced outward and upward 

 to the fenestra ovalis. in a way unknown in any other family of the vertebrates. 



The pterygoids. — These bones have the usual relations in the 

 posterior part, but the anterior end is very different in its form and 

 relations from that ordinarily found in the primitive reptiles. Near 

 the middle point the pterygoids unite with the strong basi-pterygoid 

 processes of the basisphenoid. There are strong anterior and pos- 

 terior processes, but the external process which forms a buttress for 

 the lower jaw, and is such a conspicuous feature of the skull of the 

 Pelycosauria and the Pariotichidae, is totally absent. As this exter- 

 nal process forms one of the chief points of attachment of the trans- 

 verse bone, its absence in the skull is of considerable importance in 



