NECTURUS MACULATUS. 411 
rate,” while the operculum fits tightly and 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, which projects from the inner edge of the curved bone at approximately the 
middle and thus divides it into two nearly equal portions, the anterior or quadrate, 
and the posterior, or opisthotic halves. These two halves are nearly flat, or like very 
shallow troughs, the planes of which are set nearly at right angles to one another; and 
when in place, the anterior portion is placed nearly 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 from 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 process. 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 much uncertainty and 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 in 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 work the word “ stapes” 
was used and its probable homology stated. I was 
led to a change of this view by the examination 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 Stéhr in Triton and Siredon. This was equally 
