50 ILLINOIS BIOLOGICAL MONOGRAPHS [50 



In certain regions of the roof of the lateral semicircular canal, the cartilage 

 has been resorbed, as the foramina left in the chondrocranial roof show (Fig. 

 3), and the dermal ossification together with the lateral line ossification forms 

 the protecting roof. The cavum of the inner ear and that of the lateral osseous 

 canal are separated by the perichondrial ossification of the inner surface of the 

 resorbed wall (Fig. 31). In the 60 mm. stage, endochondrial ossification has 

 appeared in some regions between inner and outer lamellae, so that the space 

 formerly occupied by cartilage is now filled with osseous trabeculae, just as in 

 the more anterior sphenotic region (Fig. 33). Toward the posterior end of the 

 roof of the lateral semicircular canal, the perichondrial lamellae, both inner 

 and outer, are present in the 32 mm. stage (Fig. 31). 



The ossification of this region of Amiurus agrees with the descriptions of 

 Schleip, Gaupp, AUis and others for the teleosts, but the name squamosal 

 employed by them should not be used alone in naming this compound bone. 

 A squamosal element is present, but the name cannot be applied to the whole 

 bone, as it consists of a pterotic ossification with a squamosal element added 

 (Fig. 31); it is more like, but not completely in agreement with the mammalian 

 temporal. 



The epiotic ossification (Figs. 3, 38), or the ossification around the dorso- 

 posterior wall of the posterior semicircular canal, has been described by various 

 authors as a perichondrial ossification. In the teleosts it is connected to the 

 post-temporal ossification of the shoulder girdle by a ligament which ossifies in 

 connexion with the outer perichondrial lamella. In places the cartilage is 

 resorbed and the lamellae form the wall. It is homologous with one of the 

 centers of ossification of the periotic cartilage of man. 



This ossification is well developed in the 32 mm. Amiurus. Inner and outer 

 osseous lamellae appear in the perichondrium of the dorso-lateral and posterior 

 walls of the posterior semicircular canal. The ligament which connects the 

 outer lamella to the post-temporal is just beginning to ossify (Fig. 38). The 

 posterior dorsal part of the cavxim for the posterior semicircular canal is filled 

 with osseous trabeculae continuous with the inner lamella. There is no resorp- 

 tion of cartilage at this stage. 



At the 60 mm. stage, the perichondrial lamellae are much thicker and cover 

 more surface. The outer lamella has extended ventrally on the wall of the 

 semicircular canal so that it meets the ascending exoccipital ossification. 

 Near the anterior end of the posterior semicircular canal roof, the cartilage has 

 disappeared and the outer lamella alone forms the wall. 



The occipital region. This part of the cranium has grown considerably 

 since the condition described for the 10 mm. larva. The cartilage forming the 

 posterior margin of the posterior fontanelle has grown forward and, together 

 with the medial edges of the otic capsules, forms the synotic tectum (Fig. 3). 

 There are ossifications on both the inner and outer perichondria of this tectum, 

 which have been described in other teleosts as the centre of the supraoccipital 



