72 IOWA ACADEMY OF SCIENCES. 



riorly they slightly overlap the synotic tectum. The mus- 

 cular crest overlapping the dorsal border of the squamo- 

 sum of the adult condition is not yet developed. 



The squamosum is an elongate plate extending obliquely 

 across the ear capsule. About two-thirds of its length is 

 applied to the ear capsule, the remainder overlaps the 

 quadrate. Beginning at a level somewhat posterior to the 

 posterior border of the operculum (stapes), it extends 

 antero-dorsally, its ventral edge slightly overlapping the 

 operculum, toward the middle line, nearly reaching the 

 lateral border of the parietal above mentioned. After 

 passing the operculum the squamosum turns and passes 

 antero-ventrally to the quadrate, its ventral border in con- 

 tact with and resting upon the columella and farther ante- 

 riorly upon the quadrate. 



Underlying the squamosum is a bone figured and 

 described by Hay as the "columellar process of the quad- 

 rate." One may be permitted to suggest that Hay is in 

 error. He describes the bone in practically the adult con- 

 dition, after it has completely coalesced with the ossifica- 

 tion of the quadrate. He fails to see that there is an artic- 

 ulation between the columella and the quadrate, and that 

 this bone lies anterior to the articulation, that is, it is not 

 applied to the columella but to the quadrate. The articu- 

 lation mentioned occurs at the level where a division of 

 the mandibular branch of the facial nerve passes dorsally 

 around the columella-quadrate bar to go to the oral (Kings- 

 bury) line of sense organs. There is no external disconti- 

 nuity in the bar, but at the point mentioned the hyaline 

 structure is interrupted by a membranous structure so 

 characteristic of articular surfaces. The bone in question 

 appears as a distinct ossification lying in the hollow of the 

 quadrate and covered by the squamosum, except at its 

 extreme anterior end. Its antero-ventral edge rests upon 

 and is closely applied to the (juadrate. Anteriorly it 

 reaches nearly to the anterior border of the latter, and pro- 

 jects slightly beyond the squamosum. Posteriorly its 

 pointed extremity reaches nearly to the level of the artic- 

 ulation in the columella-quadrate bar above described. 



