470 HENRY C. TRACY 



appearance; furthermore, examination of sections of the skull in 

 this region show a dense membrane which apparently is not true 

 bone. What this membrane may be morphologically can only 

 be determined by a developmental study of this region of the 

 cranium, but the existence of the auditory foramen in the adult 

 can hardly be denied. 



The lateral recess has heretofore received scant attention. 

 This is all the more remarkable when we consider that by means 

 of this recess the exterior is brought into direct relation through 

 the lateral-line canals with the fluid under the utriculus and so 

 with the fluid through the whole system of the perilabyrinthine 

 canals. These relations were first noted by Breschet, whose 

 descriptions and figures show them somewhat vaguely. Most of 

 the writers since have also referred to them, but only incidently 

 and briefly. The work of Ridewood ('04 a) indicates that the 

 lateral recess is a characteristic feature of the Clupeoid skull. 

 He briefly refers to the cavity in Engraulis encrasicholus. "The 

 lateral temporal groove is broad and shallow. Removal of its 

 floor exposes a fairly large cavity opening laterally by two 

 apertures. . . . This cavity is roofed by the frontal and is 

 bounded in front by the postfrontal, prootic and squamosal, 

 behind by the squamosal and below mainly by the prootic. " 

 In Coilia nasus, he describes the recess in similar terms and 

 refers to the prootic bulla as projecting ''upward into a lateral 

 vacuity of doubtful homology." He states that the squamosal 

 bulla (posterior bony capsule) is just visible in the hinder part 

 of this cavity. This description corresponds to the relations of 

 this cavity as described in this paper. Ridewood, however, did 

 not mention the recess in connection with the other species which 

 he studied. Matthews appears to have completely overlooked it. 

 De Beaufort merely quotes Ridewood's statement. 



The relations of the lateral recess are such that changes in 

 pressure as the fish swims from one water level to another may 

 be transmitted directly from the outside by way of the lateral- 

 line canals and the loose tissue in the lateral recess to the fluid in 

 the perilabyrinthine canals. Changes in hydrostatic pressure are 

 thus conveyed directly to the walls of the utriculus. The struc- 



