THE NECTURUS 13 



at the posterior end to cover this part of the inner surface 

 of the jaw. It bears no teeth. 



The splenial is a small piece of bone applied to the 

 inner surface, on the middle of the upper edge. It is not 

 in line with the upper edge of the dentary so the teeth 

 that it bears are not in line with the rest of the jaw. 



The articular is a large unossified cartilage that supplies 

 the articulation with the quadrate. The Meckelian cartil- 

 age is unossified and may be seen on the inner face between 

 the dentary and the angular. 



Braincase 



The braincase is formed of cartilage and membrane bones. 

 The following bones enter into its structure ; parasphenoid, 

 frontal, parietal, prootic, opisthotic, and exoccipitals. 

 These along with cartilaginous material complete the brain- 

 case. 



Drawing 4. Numbering posteriorly from the head, draw the sec- 

 ond, twelfth and twenty-fifth vertebrae to show the differences in the 

 vertebrae of the several regions. 



Are there structural differences between the sacral and 

 other vertebras? Where do the first haemal processes 

 appear in the caudal vertebras? The first cervical is 

 modified from the rest of the series. The centrum is 

 short and dorso-ventrally flattened with the anterior face 

 modified to articulate with the paired condyles of the 

 skull. Note the posterior end of the centrum. Is it hollow 

 at this end? Is there any indication of a notochord? Is 

 there a prezygapophysis or postzygapophysis on this verte- 

 bra? What is the shape of its neural canal? Is there an 

 odontoid process? The atlas may have joined the skull, 

 thus making the first cervical, the axis. An odontoid pro- 

 cess on the first cervical would indicate this. 



