74 A REVISION OF THE COTYLOSAURIA OF NORTH AMERICA 



fenestra ovalis. The "double fossae" above mentioned are the osseous represen- 

 tatives of the membranous ampullae at the junction of two pairs of semicircular 

 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 forwards 

 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 backwards 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, 1 suppose 

 this increased diameter to be partly normal; but it maybe 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 phaseohnus and Empedtas molar is 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 tympanic chamber, nor is it a 

 rudimental cochlea. It does not appear to be homologous with the recessus Iaby- 

 rinthi, since that cavity is not perforated by the fenestra ovalis. !t 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 forma- 

 tion. 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 

 Dtadectidce 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 produced outwards and upwards to the fenestra 

 ovalis in a way unknown in any other family of vertebrates." 



The lower jaws (plate 4, fig. 2) of specimen No. 4684 are nearly complete, 

 lacking only the articular region of the left side. There are three pairs of large 

 incisors (the median pair the larger) and eleven cheek teeth. In specimen No. 

 1076, University of Chicago, there are three incisors and twelve cheek teeth; the 

 discrepancy is probably due to the loss or obscuration of a tooth in No. 4684. 

 The incisors have a greater anteroposterior diameter than transverse and the inner 

 side shows a worn surface. The front two teeth after the incisors are small with a 

 single cusp, but rapidly increase in size posteriorly through the third, fourth, fifth, 

 and sixth; the seventh is the largest, the tenth and eleventh are abruptly smaller. 

 The shape of the tooth is well shown in the figure of the type (plate 1, fig. 6, 6a). 

 Opposite the largest teeth the jaw is widened, not only by the increase of the alveolar 

 surface but by the retreat of the tooth line from the outer edge of the jaw, which 

 remains nearly straight. The separate bones of the jaw can not be made out; they 



*See skull of E. molar is, Proc. Amer.Philosoph. Society. 1881, plate v, figs, a and b, where the fenestra is represented. 



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



JProfessor Owen has figured (Todd's Encyclopedia, art. Monotremata) a structure in Echidna, which looks remarkably like 

 that here described. This is a tubular elongation of the meatus auditorius externus with more or less cartilaginous walls. This 

 structure might be regarded as homologous with that displayed by the Empediai, could we imagine that with their diminution in size 

 in the Monotreme, the ossicula auditus had retreated within this tube preceding the membranum tvmpani, 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. 



