OSSEOUS SEMICIRCULAR CANALS OF BIRDS. 



2. DIMENSIONS OF THE CANALS. 



The right ear was the one studied in all cases, and the semi- 

 circular canals laid bare by the knife. To measure these curved 

 canals a piece of No. 50 cotton thread was drawn around each 

 and the length marked on it ,- then the exact length of the thread 

 when straightened out was ascertained. All absolute dimensions 

 are stated in millimeters. All measurements were made twice, 

 at an interval of time, in order to secure the greatest possible 

 accuracy. For units of comparison a median vertical and a 

 median basilar cranial length were employed. The vertical 

 length is the distance from the ventral face of the occipital con- 

 dyle to the large transverse muscle crest immediately above the 

 foramen magnum ; the basilar length is the distance from the 

 posterior face of the condyle to the anterior aperture of the 

 Eustachian tube as marked by bony ridges. These were taken 

 as units of comparison because they can be very accurately 

 measured, but especially because they are lengths of parts of the 

 skull that are perhaps the least subject to variation. 



In the tables the first three vertical columns give the absolute 

 lengths of the semicircular canals. The fourth and fifth col- 

 ums give the absolute basilar and vertical lengths of the skull 

 for comparison. The succeeding three columns show which of 

 the canals are largest. The ninth column gives as the " com- 

 bined measurement" of comparison the sum of the vertical and 

 basilar cranial lengths divided by two. The tenth column ex- 

 presses the sum of the absolute lengths of all three canals. And 

 the last column represents this sum divided by the combined 

 cranial measurement given in the ninth column. This last col- 

 umn, accordingly, states the sum of the lengths of the three 

 canals in relation to a definite length of the skull, and may be 

 called the "relative total length." 



A comparison of the figures given in the last column of the 

 tables shows what genera have the relatively larger, and. what 

 ones the relatively smaller canals, expressed in terms of the 

 cranial measurements. We find then the following associations 

 according to this relative sum total length of the canals : 



i.oo mm. to 1.25 mm., the one genus Pelecamis. 



1.25 mm, to 1.49 mm., Strnthio, and certain Anatidae (Anas]. 



