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ALLIS. [Vol. XVIII. 



to the overlapping anterior process of the squamosal. This wide 

 groove forms, as already stated, a postero-mesial, slightly de- 

 pressed portion of the dilatator groove, and the hind end of its 

 antero-lateral edge usually projects upward as a small angular 

 process, the process and edge marking the line of insertion of a 

 median tendon of the dilatator operculi muscle. Immediately ven- 

 tral to the hind end of the anterior edge of the groove, on the 

 lateral surface of the ossification, there is a shallow but relatively 

 large depression which forms the articular facet for the anterior 

 articular head of the hyomandibular. The long axis of this facet 

 inclines upward and backward at about 45°, and the facet, al- 

 though lying entirely in the postorbital ossification, extends back- 

 ward almost, or quite, to the anterior edge of the squamosal. 



The postorbital ossification is not traversed by any portion of 

 the lateral canals. The main infraorbital canal, as it leaves the 

 open groove between the adjoining edges of the frontal and 

 squamosal and runs forward and downward behind the eye, passes 

 first through the dermal or subdermal tissues that lie immediately 

 dorsal to the dilatator operculi muscle, and then enters a series of 

 thin and delicate bones that lie wholly external to the muscles of 

 this part of the head. The first, or most dorsal of these bones, 

 which is quite certainly the homologue of the postfrontal bone of 

 Amia, lies directly superficial to the dorsal surface of the post- 

 orbital ossification but is separated from it by the anterior portion 

 of the dilatator operculi. The postorbital ossification of Scomber 

 thus has no dermal component, and is, accordingly, the exact 

 homologue of the postorbital ossification of my descriptions of 

 Amia. It has, however, in Scomber, invaded the anterior part of 

 the region that gives articulation to the hyomandibular ; lies, to a 

 considerable extent, ventro-internal to the squamosal ; and has, as 

 seen in Fig. 4, left, entirely, the general outer level of the dorsal 

 surface of the head. It thus seems to be moving downward along 

 the side of the skull, toward a position corresponding to that occu- 

 pied by the prootic part of the temporal bone of man ( No. 5 ) . 



The ossification adjoins the squamosal behind, the petrosal ven- 

 trally and postero-ventrally, and the alisphenoid antero-mesially. 

 Dorsally it is separated from the adjoining edges of these bones 

 by distinct, but narrow lines of cartilage ; ventrally these lines of 



