516 PEOF. T. W. BEID&E ON THE 



groove on the lateral surface of the basioccipital, and, with only 

 a slight reduction in calibre, appears to terminate at the sutural 

 junction of the latter bone with the prootic. The outer lateral 

 wall of each caecum is complete, but both dorsally and ventrally 

 its tunica externa ceases by becoming firmly inserted into tt.e 

 corresponding lips of the basioccipital groove in which the 

 cfficum is lodged. Towards the anterior termination of the 

 csecum even the tunica externa of the outer wall disappears 

 through its continuity with the hinder margin of the prootic 

 bony lamina, which, anteriorly, converts the groove into a cul- 

 de-sac. The inner wall, on the contrary, is throughout formed by 

 the tunica interna alone, and, moreover, is closely adherent to 

 the sides of the basioccipital groove. 



Eeduced to a simple wall of tunica interna of extreme tenuity, 

 the terminal portion of the auditory csecura now enters the bony 

 cul-de-sac (PI. 37. fig. 4, cs.}, and, on its inner or cranial side, 

 becomes closely applied to the outer surface of the membrane 

 closing the auditory fontanelle (fig. 5, a,/.). 



V. The Ckais'ial Fontanelles and their Eelation to the 

 Auditory Organ. 



In their account of the bones of the skull in B'oiopferns 

 Pallasii, Cuvier and Valenciennes (4. p. 143) describe two in- 

 teresting vacuities in the postero-superior aspect of the cranial 

 roof. " Ces mastoi'diens ont en' avant une tres-profonde 

 echancrure, qui cerne pres des deux tiers du giand trou parieto- 

 mastoi'dien, dont les cotes du crane sont perces, une echancrure 

 du frontal posterieur contribue aussi a former le cercle de ce 

 trou. Ce grand trou, analogue a celui que nous avons observe 

 dans r Alose et dans plusieurs autres Clupees, mais beaucoup plus 

 semblable encore a ce que existe dans le Mormyre, est bouche' 

 par une couche peu epaisse de cette mucosite graisseuse, qui 

 remi)lit les cavernes du crane et sur laquelle passe la peau mince, 

 nue et sans ecailles de la tete. Par ce trou on penetre largemeut 

 dans I'interieur de la cavite du crane, et Ton voit presque sans 

 dissection, apres avoir toutefois enleve toutes ses parties externes, 

 les canaux semicirculaires superieurs, leur ampoule commune et 

 une portion du sac qui contient I'otolithe." 



This de^cl iption is in the main an accurate cue, but may never- 

 theless be supplemented in certain details. 



