424 



Royal Society 



Composition of the Skull of the Turtle. 



It has been seen that in birds the presphenoid, ethmoid, and or- 

 bitosphenoid regions are subject to singular irregularities in the mode 

 and extent of their ossification. In the turtle, not only are the 

 I)arts of the cranium which correspond with these bones unossified, 

 but its walls remain cartilaginous for a still greater extent. In 

 fact, if a vertical section be made through the longitudinal axis of a 

 turtle's skull, it will be observed that a comparatively small extent of 

 the cranial wall, visible from within, is formed by bone, and that the 

 large anterior moiety is entirely cartilaginous and unossified. The 

 anterior part of the posterior, bony, moiety of the cranial wall is 

 formed by a bone (Pt.), whose long, vertical, anterior-inferior margin 

 forms the posterior boundary of the foramen by which the third divi- 

 sion of the trigeminal nerve makes its exit from the skull. The ante- 

 rior and superior margin of the bone is very short, and articulates with 

 the parietal bone. The superior margin is inclined backwards, and 

 articulates with the supraoccipital. The posterior margin is straight, 

 and abuts against a cartilaginous plate interposed between this bone 

 and that which succeeds it. The inner face of the bone is, as it 

 were, cut short and replaced by this cartilage, whence the inferior 

 edge is also short and is connected only with the basisphenoid, and 

 not with the basioccipital. The anterior margin of the bone cor- 

 responds with the middle of the mesencephalon, while its inner face 

 presents apertures for the portio dura and portio mollis. The pos- 

 terior margin of its outer face forms half the circumference of the 

 fenestra ovalis, and it contains the anterior and inferior portions 



--^^ 



Fig. 4. — Longitudinal section of the Skull of a Turtle {Chelone mydas), exhibiting 

 the relations of the brain to the cranial walls. The dotted parts marked AS. 

 OS. FS. and Eth. are cartilaginous. 



of the labyrinth. Thus, with the exception of the absence of an 

 inferior connexion with the basioccipital, — a circumstance fully ex- 

 {dained by the persistence in a cartilaginous state of part of the bone, 

 — it corresponds in the closest manner with the petrosal of the bird. 

 I confess I cannot comj)rehend how those who admit the homology 

 of the bone called petrosal in the bird with that called ])etrosal in 



