NECTURUS MACULATUvS. 411 



rate," while the operculum Hts tightly aud exactly into the fenestra ovalis, a large 

 lateral opening in the cartilaginous otic capsule. 



The most conspicuous character of the paraquadrate is the short and blunt opercular 

 process, whicli projects from the inner edge of the curved bone at approximately the 

 middle and tlius divides it into two nearly equal portions, the anterior or quadrate, 

 and the postericn-, or opisthotic halves. These two halves are nearly flat, or like very 

 shallow tioughs, the planes of which are set nearly at right angles to one another; and 

 when in place, the anterior portion is placed neai'ly perpendicularly to the skull over- 

 lapping the quadratum along the side, while the posterior portion is nearly horizontal and 

 covers the outer part of the dorsal surface of the opisthotic. 



The paraquadrate is always connected with three bones and may possibly come in 

 contact with two more. By its opercular process it forms a definite articulation with the 

 columellar process of the operculum, and its two flat portions are applied to the outer sur- 

 face of the quadrate and opisthotic. Aside fi'om these, its anterior and posterior ends 

 may touch the outer corners of the palato-pterygoid and parietal respectively. If an 

 isolated paraquadrate be held so that the opercular process is directed downwards, it is in 

 its normal position and the position of the planes of the anterior and posterior portions 

 will serve to locate it. 



The operculum fits something like a stove lid into the fenestra ovalis, an oval open- 

 ing in the cartilaginous otic capsule. It consists of a flattened oval base or body bearing 

 upon its outer surface an irregular columellar jjrocess. By means of this process it articu- 

 lates with the opercular process of the paraquadrate and normally touches no other bone, 

 although in old animals the pro-otic bone may enlarge sufficiently to come in contact with 

 its anterior edge. The columellar process is directed upwards and a little forwards, and 

 will thus give the proper orientation for the bone. 



The homologies and, consequently, the nomenclature of these two bones have been a 

 matter of nnich uncertainty aud varient treatment among authors. For the first of these 

 I have selected the term '' paraquadratum " on the authority of Gaupp, who has proposed 

 it as at least a provisional term to indicate the dermal encasing piece associated in the 

 amphibians with the quadrate. He apparently inclines to the belief that this element 

 may prove homologous with the mammalian tympanicum, a term by which he designates 

 the piece ill his revision of the " Anatomie des Frosches." The operculum has become so 

 universally identified with the stapes of higher forms that in the first writing of the manu- 

 script for this w<n-k tiie word " stapes" wa used and its probable homology stated. I was 

 led to a change of this view l)y the exaiuination of a set of slides of a larval Necturus 

 of 44 mm. in which the part in question arises as a semi-detached bit of the cartilaginous 

 otic capsule, precisely as was seen by Stohr in Triton and Siredon. This was equally 



