394 MK. W. p. PYcBAi'T ox I'UE [Mar. 21, 



narrow synchondrosis widening slightly forwards. Its dorsal 

 border is embedded in a mass of cartilage lodging the procitic, 

 and dividing the exoccipital from the squamosal. Its anterior 

 border looLs somewhat upwards and outwards, and is continued 

 dorsalwards into the prootic cartilage, whose free edge forms the 

 posterior wall of the tympanic recess. Seen from withiu, the 

 exoccipital is more or less flabellate anteriorly, with an elongate 

 posterior stem. The inner segment of its convex border abuts 

 against the basioccipital, the outer is bounded by the opistholic. 

 The stem is bounded on one side by the vagus foramen, on the 

 other by the foramen magnum. 



The supraoccipital is couipletely ossified iuferiorly and separated 

 from the exoccipital by synchondrosis. Its superior border is as 

 yet very incomplete, deeply concave, with a crenated free edge. 

 Thus a large lambdoidal or parieto-occipilal fontanelle is formed. 

 Its dorso-lateral angle joins the parietal, its ventri-lateral the 

 exoccipital, by means of a short, narrow bar ; between these two 

 areas is a wide chink, separating the supraocci])ital from the 

 epiotic. The groove lying below this chink is scooped out of 

 the thin plate of bone joining the epiotic to the supraoccipital. 



The epiotic, seen from without, is represented by a subcrescentic 

 tract of bone, bounded along its iuner border, above by a wide 

 chink, and below by the upper part of a deep groove from the 

 supraoccipital. The upper end of its outer border is embedded 

 in the prootic cartilage, its lower end is separated by a thin band 

 of cartilage from the exoccipital. Seen from within, it takes the 

 form of a perfectly free semicircular coil bounding the iioccular 

 fossa posterioi'ly. Its upper and lower ends are separated by 

 cartilage from the prootic. It is fused with the supraoccipital 

 by means of a narrow plate of bone extending from the posterior 

 border of its inferior end. (PI. XXIII. figs. 1, 2.) 



The prootic, from the outside, appears as a broad oblong tract 

 of cartilage lying between the squamosal and exoccipital. Its free 

 border forms the posterior wall of the tympanic recess, and is 

 continuous with that of the squamosal prominence. It is 

 bounded posteriorly by the epiotic. The floccular fossa, at this 

 stage, lies in this tract of cartilage, in the angle between the 

 squamosal and parietal above, and the exoccipital and epiotic 

 below. 



On the inside, it is bounded by the epiotic behind, and the opisth- 

 otic below. Between its junction with the epiotic and the opisthotic 

 its border is deeply excavated to form the outer boundary of the 

 floccular fossa. Its supero-lateral border rests upon the lower 

 end of the squamosal, and cuts off this bone fi'om particij^ating in 

 the formation of the brain-case. It is bounded on either side by 

 the ]jarietal (behind) and the alisphenoid (in front) ; its anterior 

 border is bounded in part by the alisphenoid, and in part by a 

 mass of cartilage lying between this and the basisphenoid, which 

 probably represents tissue into which ossification was destined to 

 spread from the alisphenoid, prootic, and basisphenoid. The 



