THE STRUCTURE OF THE COLUMELLA AURIS IN THE PELYCOSAURIA 



READ OCTOBER 16, 1SS4. 



By E. D. Cope. 



In a specimen of the rermiau reptile Clepsydrops leptocephalvs Cope,* the columella auris was 

 found nearly in its normal position. It was found lying on the internal side of the normally joined 

 squamosal and quadrate boues, the greater part of it within the former, but the distal extremity 

 overlapping the superior part of the latter. These elements have lost their attachment to the 

 cranium proper, so that the connection of the columella with the latter is not visible. 



The columella is of unusual size as compared with other bones of the skull. Thus while the 

 vertical length of the premaxillary bone is M. .000, and its width at the third tooth is .022, and 

 while the vertical length of the quadrate bone is .085, the dimensions of the columella auris are 

 as follows : 



Length on inside of curve 072 



Greatest diameter just below stapes 021 



Distal diameters \ ^ 014 



< short 011 



Diameters of head of epicolumella < ^ 



( short .- 0095 



Diameters of disk of stapes} l ° us 029 



1 I short 021 



The shaft is slightly curved. The proximal extremity is divided by a fissure which is at right 

 angles to the long transverse diameter. The smaller of these divisions is the more prominent, and 

 its free extremital angle is formed by the continuous concave edge of the shaft. It bears the same 

 relation to the shaft as the head of a rib does to its shaft (Fig. 1). The other proximal division 

 occupies the position with reference to the shaft that the tubercle does to the rib. It is much larger 

 than the inner head of the columella, and its face looks away from that of the head at an angle of 

 120°. Its long diameter diverges from that of the head by an angle of about 145°. Its free surface is 

 a wide oval, and is concave, forming a basin-shaped lid to the foramen ovale of the internal ear. It 

 thus represents the expanded proximal extremity of the stapes of other vertebrates. The base of 

 this stapedial portion is perforated in the direction of its long diameter by a canal. Oue foramen 

 of this canal is situated on the external edge below the external extremity of the oval basin. The 

 other foramen issues in a groove, which continues for a short distance on the inner side of the bone 

 from the Assure which separates the epicolumella from the stapes. This canal is, no doubt, that for 

 the mandibular artery, and represents the foramen of the stapes, which is present in many Mam- 

 malia (Fig. 1 e e). 



The distal extremity of the shaft is concave, and shows an articular surface of ridges and pits 

 (Fig. 1 c). The coarseness of the latter indicates that the distal element attached at this point was 

 cartilaginous, at least at the point of attachment. It will then resemble the corresponding part in 

 the Orocodilia and Lacertilia, which connects the columella with the membranum tympaui. 



The points above determined as to the structure of this element permit of a number of inter- 

 esting deductions. 



First. This columella possesses what has not been previously observed in reptiles and higher 



* Proceedings American Philosophical Society, 1884, p. 30, 



