DEVELOPMENT OF THE SKULL OF EMYS 753 



The parasphenoideum lies immediately in front of and par- 

 tially embraces on the two sides the stalk of the hypophysis at 

 this stage. It is also of interest that a small lamella of bone 

 extends horizontally forward from the front margin of the crista 

 sellaris to occlude partially the fenestra hypophyseos. In an 

 older embryo having a carapace length of 16 nam. this lamella 

 from the crista sellaris has extended itself fm-ther forward and 

 fused completely with the parasphenoideum so that only a very 

 small opening is left between the posterior processes of the 

 parasphenoideum and this lamella for the accommodation of the 

 hjrpophysial staUc. In the meanwhile the ossification of the crista 

 sellaris, which becomes the basisphenoideum, has set in. 



The basisphenoideum of the adult Emys must therefore be 

 composed of two parts, an anterior part not preformed in car- 

 tilage and a posterior part laid down originally in the crista 

 sellaris as a replacing bone. 



The vomer (».) in the stage modelled is an unpaired bone hav- 

 ing the form of a shallow trough which lies with its dorsal concave 

 face ventral to the anterior end of the septum interorbitale and 

 separated from it by a thin layer of connective tissue. In form 

 the vomer tapers shghtly in a horizontal plane toward the ante- 

 rior end and also grows thicker dorso-ventrally. 



In an embryo having a carapace length of 8.5 mm. the vomer 

 shows clearly its paired nature, being represented by a pair of 

 thin lamellae set at an angle to each other so that they fit about 

 the ventral edge of the septum interorbitale between the pos- 

 terior opening of the ductus naso-pharjmgeus behind and the 

 cartilago paraseptalis in front. In an embryo having a cara- 

 pace length of 13.5 mm. the vomer extends relatively fiu-ther 

 posteriorly than in the stage modelled and its lateral margins 

 Ue close to the median margins of the two palatina which, however, 

 come to lie rather dorsal to the vomer. 



The investing bones of the adult lower jaw are all present 

 in the stage modelled except the complementare which appears 

 first in an embryo having a carapace length of 13.5 mm. They 

 may be designated as the dentale, angulare, supra-angulare, and 

 goniale. An operculare is lacking as in the adult. The arti- 



