TABIE 4-2 THE BONES OF THE HEAD SKELETON OF THE REP- 

 TILE ARRANGED ACCORDING TO TYPE AND PLACE 

 OF ORIGIN 



The floor of the skull is covered by an elongated para- 

 sphenoid which is fused with the basisphenoid. The basi- 

 sphenoid has strong basipterygoid processes articulating 

 with the pterygoids. Parabasal canals pass between the 

 basisphenoid and parasphenoid on either side. This canal 

 gives passage to the internal carotid artery and the palatine 

 branch of the seventh nerve. 



In the membranous anterolateral wall of the cranium is 

 a small y-shaped bone, the orbitosphenoid. The bones of 

 either side do not fuse ventrally in a "presphenoid union." 

 There are small septomaxillaries visible in the opening of 

 the external naris. These bones overlie the Jacobson's 

 organs opening through the incisive foramina of the pala- 

 tal surface, and they are of both dermal and endochondral 

 origin. The eye is surrounded by a ring of about fourteen 

 sclerotic plates. 



The upper jaw consists of separate premaxilla and maxil- 

 la. These bear conical teeth. The palatoquadrate complex 

 differs in having large palatal and interpterygoid fenestrae, 

 confluent incisive fenestrae, and internal narial openings, 

 and also differs in its component bones. The palatine sutures 

 anteriorly with a large vomer; behind the palatine are 



pterygoid, ectopterygoid, and quadrate. From the dorsal 

 side of the pterygoid, an epipterygoid extends upward to 

 contact the side of the cranium. The V'l branch of the 

 trigeminal nerve passes medial to this bone, whereas the V2 

 and V3 branches pass outward behind it (Figure 4-2 C). 

 The pterygoid bears a row of teeth in Iguana. 



The lower jaw (Figure 4-2 D,E) is composed of seven 

 bones rather than of the dentary alone. The dentary bears 

 conical teeth similar to those of the upper jaw. The teeth 

 show some variation from front to rear and some of them 

 have three cusps. A small mental bone may occur in 

 Meckel's cartilage at the anterior symphysis, or joint. 



The hyoid arch consists of the stapes above and a part of 

 the hyoid apparatus below. The stapes is a long slender 

 bone with an ovoid foot plate fitted into the fenestra vesti- 

 buli of the otic capsule. Distally it attaches through the ex- 

 tracolumella to the paroccipital process above, the tympanic 

 membrane laterally, and the quadrate anteroventrally (Fig- 

 ure 4-3). The stapes plus the extracolumella forms the 

 columella. 



The hyoid apparatus consists of a copula ("joining piece") 

 with a long lingual (or entoglossal) process. This process 

 is detached in the case of Tupinambis (Figure 4-3). Parts of 

 three pairs of arches attach to the copula. The only ossifi- 

 cation in the hyoid apparatus is that of the ceratobranch- 

 ials I, although other areas may be well calcified. 



EMBRYOLOGicAL DEVELOPMENT Of the lizards, the devel- 

 opment of Lacerta agilis is best known (Figures 4-4 to 4-6). 

 The first structures to appear include Meckel's cartilage, 

 the widely separated trabeculae, a parachordal plate formed 

 by connection of the parachordals below the notochord, 

 and, fused to this, an occipital arch on either side. In the 

 otic capsule a center of chondrification appears laterally 

 and gradually spreads out. The three hypoglossal roots be- 

 come enclosed from behind forward by the appearance of 

 preoccipital arches. The trabeculae join anteriorly as a 

 trabecula communis but remain separated posteriorly from 

 each other and from the parachordals. The palatoquadrate 

 is represented by a quadrate rudiment and a separate cres- 

 centic-shaped piece formed of the pterygoid and ascending 

 processes. The hyoid apparatus consists of basihyals, form- 

 ing a lingual process and body (copula) and hypohyal, 

 ceratohyal, ceratobranchial I and ccratobranchial II arch 

 centers — the latter appear as bilateral processes from the rear 

 of the copula. 



The definitive chondrocranium is quite different in ap- 

 pearance from that of the mammal but contains many of 

 the same elements (Figures 4-5, 4-6). 



The development of the head bones is known from several 

 other lizards but not for Lacerta. In contrast to the mammal, 

 the otic capsule is ossified from two centers and postparietal 

 centers are never present. The nature of the epipterygoid 

 and the orbitosphenoid as palatoquadrate and cranial wall 

 elements respectively is clearly revealed. 



64 • HEAD SKELETON OF OTHER TETRAPODS AND CHOANATES 



