626 J. PLAYFAIR MOMUilRICH. 



length, projecting out into the integument. Still move poste- 

 riorly only one such plate of membrane is seen, which occupies 

 a median position at the vertex. These a})pear to be the 

 membranous rudiments of the frontals and dermo-supra- 

 occipital. 



The Adult. — In the adult cranium the cartilage persists to a 

 great extent, although surrounded almost completely by bone. 

 No enchondroses, as in the higher vertebrates, appear ; ectostoses 

 and parostoses form the cranium. One point noticeable at the 

 first glance is the elongation of the occipital regions, and the 

 compactness of the region immediately behind the orbits ; 

 separating these two portions there is a membranous space, 

 closed in by the parasphenoid. 



The occipital region ossifies below as the basi-occiptal (fig. 

 9, B. O.). Posteriorly this is round, forming an articular facet 

 for the first vertebra, but anteriorly it becomes flattened out, 

 and expanded into a thin plate — being, in fact, fan-shaj)ed. 

 Posteriorly, on section, the rapidly diminishing notochord is 

 plainly visible, and on either side of this are the parachordal 

 cartilages. More anteriorly the chorda does not appear, and 

 the parachordals unite to form a single plate. Below it is 

 deeply grooved for the reception of the parasphenoid, which 

 underlies it, and is almost enclosed by it. Laterally it articu- 

 lates with the exoccipitals, and more anteriorly with the 

 pterotics. 



The exoccipitals (Ex. O.) are well-developed bones, forming 

 the postero-lateral floor of the skull, and extending somewhat 

 upwards upon its sides to articulate with the epiotics, and in 

 front with the pterotics. Above them on either side is a 

 parosteal bone (fig. 8, S. Tp.), upon whose homologies I am 

 undecided. I am inclined, however, to consider it homologous, 

 to a certain extent at any rate, with the supra-temporal of 

 Ami a. 



Above, at the vertex of the skull, is the supra-occipital 

 (S. O.), a large bone, extending forward nearly to the sphenotics. 

 It appears to consist of two portions; (1) a parostosis, which 

 may be termed the dermo-supra-occipi tal, and which de- 



