Nos. IAND2.] AXATOMY OF SCOMBER SCOMBER. ^q 



of its dorsal surface there is a median, slightly depressed region 

 which extends from near the front end of the skull backward 

 approximately to the plane of the hind end of the orbit. Slightly 

 posterior to the middle of the orbit there is usually a strong, and 

 sometimes almost pit-like depression, the deepest part of which 

 lies immediately in front of the intracranial, epiphysial ridge and 

 directly above a recess of the cranial cavity that lodges the fore 

 brain and the olfactory lobes. Lateral to, or postero-lateral to 

 the depression, on each side, near the lateral edge of the skull, 

 three, deep, longitudinal grooves begin, and, widening as they 

 extend backward, soon occupy nearly the entire dorsal surface of 

 the skull. 



These three grooves, on each side of the head, are separated 

 from each other by tw'O tall, thin ridges of bone, and the posterior 

 half or two thirds of the mesial groove, on each side, is separ- 

 ated from the corresponding groove of the opposite side of the 

 head by a similar ridge. This latter ridge, which is median in 

 position, forms an anterior prolongation of, or is the anterior end 

 of, the strong spina occipitalis. The other two ridges, on each 

 side, project upward and laterally, the more lateral one of the two 

 inclining laterally, much more than the other.' This more lateral 

 one of the two ridges is continued posteriorly by a sharp, flat pro- 

 cess of the squamosal, the superficial edge of the process lying, 

 however, at a deeper level than the outer edge of the ridge in front 

 of it, a sharp angle usually separating them. This angle forms 

 the hind end of the ridge proper, and the outer edge of the ridge 

 is here always enlarged, the enlargement marking the place where 

 the preoperculo-mandibular sensory canal joins the main infra- 

 orbital one. 



The lateral one of the three grooves thus found on each side of 

 the head, lodges the dilatator operculi muscle. The other two lodge 

 anterior extensions of the trunk muscles. The lateral groove is the 

 dilatator groove of Sagemehl's descriptions of other fishes (No. 

 65, p. 61), while the next adjoining one has the lateral and 

 mesial boundaries of his temporal groove (No. 65, p. 80) but dif- 

 fers radically from that groove in not being roofed by the dermal 

 bones of the skull. The name temporal groove can, however, be 

 retained for it. The mesial groove, although not specially de- 



