clemys temminckt, the Alligator Snapping Turtle; Chelydra ser- 

 pentina, the Snapping Turtle; and Terrapene Carolina, the 

 Box Turtle. 



The solid roof may be emarginated posteriorly (Figure 

 4-9). In Terrapene it is largely missing, except for a lateral 

 squamosai-postorbital arch. The parietal forms a part of the 

 endocranial wall, extending down in front of the trigeminal 

 nerve to contact the palatoquadrate (pterygoid) — the Vi 

 branch passes medial to this process, which lies in the posi- 

 tion of the lizard epipterygoid. There has been extensive loss 

 of bones in this type of skull: the lacrimal, postfrontal, su- 

 pratemporal, postparietal, septomaxilla, and splenial are 

 lacking. The nasal, ectopterygoid, and epipterygoid are 

 usually lacking; the epipterygoid is at best rudimentary and 

 the ectopterygoid may never be present. The parasphenoid 

 is lacking or much reduced. 



Other peculiarities are as follows: the vomers fuse medially 

 and suture with the pterygoids; the posterior margin of the 



quadrate is deeply incised or encloses the shaft of the stapes; 

 and the jaw margins are toothless and covered by a horny 

 sheath. The hyoid apparatus consists of a plate-like copula 

 to which are attached hyoid cornua (horns) and ceratobran- 

 chial I and II processes. 



The development of this type of head skeleton shows a few- 

 modifications. No bones other than those observed in the 

 adult are evident during developmental stages. The vomer 

 develops from bilateral centers but these soon fuse. Two 

 parasphenoid centers appear in the sea turtle, Lepidochelys, 

 but disappear as the pterygoids meet below the basi- 

 sphenoid, obliterating the basipterygoid processes and closely 

 investing most of the basisphenoid. An anterior process of 

 the supraoccipital is present. 



The head skeleton of the turtle appears to be modified 

 in almost every respect. Further evidence of the simplifica- 

 tion (or inherent simplicity) of this type is the lack of a 

 pouched Jacobson's organ and the lack of conchae, folds 



foramen apicale 



eprphanial foramen 



part of olfactory nerve 

 serving Jacobson's organ 



oculomotor nerve (III) 



trochlear nerve (IV) 



pila antotica 

 basitrabecular process 



internal carotid -, — . 



epipterygoid-y^ 



abducens nerve 



trigeminal (V) root 



motor division of V. 



notochord 



quadrate 



ophthalmicus profundus (V,) 



anterior cerebral artery 

 mandibular branch, V3 

 ophthalmic artery 



maxillary branch, V2 



middle cerebral artery 

 chorda tympani 



facial nerve root 

 stapedial artery 



columella 



hyomandibular branch VII 

 tip of ceratohyol 



lateral head vein 



synotic and posterior tectum 



occipital vein 



Figure 4-6. Relationships of nerves and blood vessels to the definitive chondrocranium as seen in 

 dorsal view. (After Goodrich, 1 930) 



OTHER TETRAPODS 



69 



