408 



HARRIS HAWTHOliXE WILDER ON 





and median, uiid lodge, respectively, the horizontal and posterior vertical senii-rinnihir 

 canals of the membranous labyrinth. The ampullae of these two canals lie at their 

 posterior union and are situated in the deep recess. The large central depression forms a 

 portion of the median chamber which contains the nu'iuliranous \estibule with its large 

 otolith. The opisthotic comes in contact with two bones, the paracjuadrate and the exoc- 

 cipital. The former merely overlaps with its posterior portion the outer margin of the 

 opisthotic, but the exoccipital forms, at least in large specimens, a definite union tlirough 

 the medium of distinct processes projecting from the two bones. 



The side to which a given opisthotic bone belongs is best determined by the canals 

 and the deepest recess. The canals lie nearest to the dorsal surface and the recess is on 

 the internal side. 



3. EXOCCIPITAL. This pair of bones, the bodies of which form the occipital condyles, 

 embraces and defines the foramen magnum and, with the addition of dorsal and ventral 

 cartilaginous arches, entirely encloses it. Each bone consists of a body and two flat 

 processes, the supra- and basi-occipital alae. The body is practically identical with the 



condyle, and its posterior aspect forms 

 a rounded surface for articulation with the 

 atlas. The dorsal or supra-occipital ala 

 is nearly perpendicular to the rest of the 

 bone and to the floor of the skull and is 

 set obliquely, at an angle o about 45° with 



both longitudinal and transverse axes of 

 the head, so that, when viewed from above, 

 the median occipital region forms a con- 

 spicuous re-entrant angle. This ala arises 

 from the body as a narrow stalk which rap- 

 idly widens, and thus resembles a triangle resting upon its apex. The dorsal margin, or 

 base of the triangle, is grooved for the reception of the cartilaginous arcus supra-occipi- 

 talis which spans the median interval between this ala and its opposite. 



The ventral or basi-occipital ala lies exactly in tlie horizontal plane, coincident with 

 that of the parabasale, and is applied so closely to this latter bone by its ventral surface 

 that the separation is often attended with some little difficulty. Both W. K. Parker and 

 Wiedersheim found the exoccipitals in Proteus actually a part (jf the parabasal, or, as the 

 latter expresses it, " synostotisch verbunden." This is nearly the case in Necturus, but, as 

 the two bones are really separable in all cases tried, it is possible that this may be the 

 actual condition in Proteus also. The ventral ala is triangidar in shape but is attached to 

 the bodv of the bone by a side and not an angle, and, when ^dewed from the ventral side. 



v.y. 



POSTCRIOR.. 



VENTRAL. 



Fig. 10. Two views of the right exoccipital. X 3. 



