47] THE SKULL OP AMIURUS— KINDRED 47 



cavum of the anterior dorsal end of the skeletal anterior semicircular canal is 

 filled with osseous trabeculae extending between the osseous lamellae in the 

 perichondrium of its walls (Fig. 36), The outer lamella extends down on the 

 side wall of the capsule as far as the hyomandibular articulation, but does not 

 enter into the formation of the articular surface, which is as yet of cartilage. 

 The abductor hyomandibularis muscle and the levator opercuH are attached 

 to the outer lamella, the former having the broader surface of attachment. 



A part of the cartilage in the roof of the anterior semicircular canal recess 

 has been resorbed together with the outer perichondrial lamella, so that the 

 inner lamella forms the wall between the cavum of the recess and the lumen of 

 the lateral line osseous canal (Fig. 19). This is just the reverse of the process 

 that Schleip observed in Salmo, where the inner perichondrial lamella disap- 

 peared first. Toward the posterior end of the hyomandibular articular surface, 

 the outer perichondrial lamella disappears and the lateral line ossification alone 

 remains; it is separated from the underlying cartilage by fibrous connective 

 tissue. This is the posterior end of the sphenotic, and when the perichondrial 

 lamella appears again, it is on the dorsal surface of the lateral semicircular canal 

 region of the otic capsule and is the centre of the pterotic ossification. 



In the 60 mm., stage, cartilage resorption in the roof of the anterior semicir- 

 cular canal recess has gone still farther, the outer and inner lamellae are indis- 

 tinguishably fused with each other (Fig. 3Z), and endochondrial ossification has 

 occurred, so that the place originally occupied by cartilage is now filled with 

 osseous trabeculae. The cartilage near the posterior end of the roof of the recess 

 still persists, and a thicker and spongier osseous mass projects laterally above 

 the hyomandibular facet. Behind this, as in the 32 mm. stage, the lateral line 

 ossification of the surface of the sphenotic passes on to the surface of the squa- 

 mosal part of the succeeding ossification. 



The ossification surrounding the lateral wall of the recess for the lateral 

 semicircular canal, is the squamosal of most authors and the pterotic of Parker 

 (1873). As part of the bone is preformed in cartilage and part of it of mem- 

 branous origin a combination of these two names is preferable as it impUes 

 relationships which neither term used alone would signify. Hence I propose 

 the name squamoso-pterotic for this mixed ossification. 



AlUs (1899) made a detailed study of the descriptions of this bone in differ- 

 ent groups of fishes by the investigators before him. He says, m summarizing 

 his review: ''We thus see that the squamosal of fishes is composed of a canal 

 component and a deeper lying component which may be either a so-called 

 membrane bone, or such a bone fused with a so-called primary ossification. 

 The primary ossification may be wholly wanting, and perhaps the canal com- 

 ponent also. Furthermore the canal component may be found entirely separated 

 from the underlying bone, may be found fused simply with an underlying 

 membrane component, or may be fused with such a component and with a 

 so-called primary ossification, which latter ossification alone is traversed by the 



