ANATOMY OF VERTEBKATES. 14i) 



relative size and position, tlie alisphenoids agree with those of the 

 Frog. Each alisphenoid is a thick suboval piece, with a tuber- 

 cular process on its under and lateral part : it rests upon the 

 basisphenoid and basioccipital, sup])orts the posterior part of the 

 parietal and a portion of the mastoid, 8, and unites anteriorly with 

 the descending lateral plate of the parietal bone. 



riie parietal, 7, is a large and long, symmetrical roof-shaped 

 bone, with a median longitudinal crest along its up]ier surface, 

 wdiere the two originally distinct moieties have coalesced. It is 

 narrowest posteriorly, where it overlaps the superoccipital, and is 

 itself overlapped by the mastoid : it is convex at its middle part on 

 each side the sagittal spine, and is continued downward and in- 

 ward to rest immediately upon the basisjjhenoid, 5. This j^art of 

 the parietal seems to 1)6 formed hj an extension of ossification 

 along a membranous space, like that which permanently remains 

 so in the Frog, between the alisphenoid and orbitosphenoid : the 

 mesencephalon and the chief part of the cerebral lobes are protected 

 by this unusually developed spine of the mesencephalic vertebra. 

 The optic foramina are oonjugational ones, between the anterior 

 border of the lateral plate of the ])arietal and the posterior border 

 of the corresponding plate of the frontal. 



The frontals, ii, rest by descending lateral plates, representing 

 connate orbitosphenoids, upon the presphenoidal prolongation of 

 the basisphenoid : the upper surface of each frontal is flat, sub- 

 quadrate, broader than long in the Boa, and the reverse in the 

 Python, where the roof of the orbit is continued outward by a 

 detached superorbital bone : there is a distinct, oval, articular sur- 

 face near the anterior median angle of each frontal to which the pre- 

 frontal; 1 4, is attached : the angle itself is slightly produced to form 

 the articular process for the nasal bones. The smooth orbitosphe- 

 noidal plate of the frontal joins the outer margin of the upper surface 

 of the frontal at an acute angle ; the inner side of each frontal is 

 deeply excavated for the prolongation of the cerebral lobes, and 

 the cavity is converted into a canal by a median vertical plate of 

 bone at the inner and anterior end of the frontal. The frontals join 

 the parietals and postfrontals behind, and, by the connate orbito- 

 sphenoid plates, the presphenoid below, the prefrontals and nasals 

 before, and the superorbitals at their lateral margins. The orbito- 

 sphenoidal plates have their bases extended inward, and meet below 

 the prosencephalon and above the presphenoid, as the neurapo- 

 ijhyses of the atlas meet each other above the centrum. The anterior 

 third part of such inwardly produced base is met by a downward 

 production of the mesial margin of the frontal, forming a septum 



