A TURTLE 147 



The hyoid apparatus consists of (1) a median flattened body, 

 usually a cartilaginous plate the dorsal surface of which is concave 

 and holds the larynx, and (2) two pairs of lateral projections, or 

 cornua, which are usually partly of bone. At the lateral border of 

 the anterior portion of the body of the hyoid on each side is also, 

 in many turtles, a small cartilaginous projection. 



Exercise 23. Draw the hyoid apparatus. 



Thoroughly clean the remainder of the skull ; in removing the 

 tympanic membrane note the columella, a minute rod of bone 

 which extends directly inward from it. Observe the general char- 

 acter of the skull. The cranial bones, which form the greater part 

 of it, and the visceral bones, which form the upper jaw and a part 

 of the roof of the mouth, are firmly joined by sutures and together 

 form a very strong and compact whole. 



Observe the nostrils, or external nares, at the anterior end, and 

 the internal nares in the roof of the mouth ; also the circular orbits. 

 Note at the hinder end of the skull the large foramen magnum and 

 the long bony spine projecting above it. Note that the brain cav- 

 ity is much narrower than the whole skull, which is very wide pos- 

 teriorly ; and that on each side, just back of the orbit, is an ex- 

 tensive arch which partially covers a space called the temporal 

 fossa. This fossa is continuous with the orbit in front and extends 

 backward as a wide groove ; it contains the powerful muscle which 

 closes the lower jaw. Note the large external ear opening, and the 

 position of the tympanic membrane. Note the condyle by which 

 the skull articulates with the vertebral column ; also the articular 

 surface of the lower jaw. 



Study first the dorsal surface of the skull. Beginning at the 

 anterior end, the medial bones are the paired prefrontals, frontals, 

 and parietals, and the median supra-occipital, the latter bone bear- 

 ing the long spine which projects back from the base of the skull. 

 Identify these bones. Lateral to the medial bones on each side 

 are the following : the maxilla, which forms the lower and the an- 

 terior border of the orbit; the postfrontal, which adjoins the 

 frontal and forms the upper portion of the posterior border of the 

 orbit ; the jugal, which is just ventral to the postfrontal and forms 



