A TURTLE 149 



Cut away the bony arch at the left side of the skull and expose 

 the temporal fossa. Identify the bones which have already been 

 observed. Along the ventral margin of the parietal note the small 

 alisphenoid bone. The large foramen at the inner side of the quad- 

 rate is for the passage of the trigeminal nerve. Note that the two 

 orbits communicate with each other, not being separated by a 

 bony septum ; the membranous septum which was present is usu- 

 ally destroyed in the cleaning. Note also that the anterior end 

 of the brain case is not bony. 



Exercise 26. Make a drawing, on a large scale, of the lateral aspect 

 of the skull. 



Study the posterior aspect of the skull. Identify the bones and 

 other features already mentioned. At the side of the condyle are 

 two foramina, the nearer and smaller of which is that of the 

 hypoglossal nerve and the larger that of the vagus and the spinal 

 accessory nerves. 



Exercise 27. Make a drawing, on a large scale, of the posterior aspect 

 of the skull. 



Of the bones just studied the following belong to the visceral 

 skeleton, forming the upper jaw and the suspensorium of the lower 

 jaw: the premaxillae. maxillae, jugals, palatines, pterygoids, quad- 

 ratojugals, quadrates, and squamosals,— of which the only carti- 

 lage bones are the quadrates. The remaining bones form the 

 cranium ; they fall into two categories,— those of the cranium 

 proper, or brain case, and those of the special-sense capsules. 



The bones of the cranium proper fall rather easily into three 

 groups : a posterior group, comprising the occipital bones ; a mid- 

 dle group, comprising the basisphenoid, the alisphenoids, and the 

 parietals ; and an anterior group, comprising the frontal bones. Of 

 these the occipital bones and the basisphenoid are cartilage bones, 

 the others being membrane bones. The presphenoid, orbitosphe- 

 noid, and ethmoid elements of the skull remain cartilaginous in the 

 turtle and thus will not appear in a skull that has been cleaned. 



The special-sense capsules are the auditory, optic, and nasal. 

 These capsules, which are entirely cartilaginous or membranous in 

 the embryo, remain largely in this condition throughout the life 



