io6 ALUS. [Vol. XVIII. 



anterior to this articular facet there is a small ridge of bone, 

 usually ending posteriorly in a sharp process. The ridge projects 

 downward, backward and laterally, and slightly overhangs a slight 

 groove in which lie from two to five small foramina. These foram- 

 ina, which transmit the roots of the three occipital nerves, have 

 already been referred to in one of my earlier works (No. 6) and 

 will be again referred to in describing the occipital nerves. The 

 posterior process of the ridge gives insertion to the third inter- 

 muscular septum of the trunk muscle. A slight process often 

 found near the anterior edge of the ridge gives insertion to the 

 second intermuscular septum. These processes are, so far as can 

 be judged from their general appearance, simply membranous for- 

 mations developed in connection with the septa to which they give 

 insertion. Nothing, excepting the fact that they give attachment 

 to intermuscular septa, indicates, in any way, that they are the 

 homologues of the two occipital arches of Amia. On the con- 

 trary, they seem, in every respect, to replace or mark, in Scomber, 

 those eminences on the cartilaginous occipitale laterale of larvae 

 of Amia, which, in that fish, were said by me (No. 4, pp. 725- 

 727) to mark, as they do also in Scomber, the lines of insertion of 

 the second and third intermuscular septa. 



The anterior edge of the occipitale laterale is irregular in out- 

 line. In certain places it is thick, in others thin, being, in the 

 latter places, cut away by the recess for the hind end of the sinus 

 utriculi posterior, and by those ends of the external and posterior 

 semicircular canals that take their origins from that sinus. The 

 recess for the sinus utriculi lies immediately dorso-anterior to the 

 vagus foramen, and is separated from that foramen by only a thin 

 plate of bone. The hind end of the external semicircular canal 

 cuts into that thick edge of the occipitale laterale that separates 

 its ventro-lateral and dorso-posterior surfaces. The ventral end 

 of the posterior semicircular canal lies directly internal to the 

 ridge that separates the dorso-posterior surface of the bone into 

 two parts or regions. The thick parts of the edge of the bone are 

 somewhat cartilaginous in their central portions, the mesial end 

 of the anterior edge of the bone enclosing, with the adjoining 

 bones, a distinct nodule of cartilage. 



