COMPARATIVE ANATOMY 17 



The upper jaw, unlike that of the shark, is Inseparably fused 

 with the cranium and must be considered as part of It. Because 

 of their manner of origin from the dermis, the superficial bones 

 of the cranium and jaws are known as dermal, membrane, or 

 investing holies. Remember that the dermal bones are on the 

 outside, embedded in the skin, and that a typical cartilaginous 

 skull is present inside the covering of scales. 



A study of the Individual bones will not be attempted. 

 Notice, however, that they are very numerous. Development 

 of membrane bones was greatest in the earliest amphibians and 

 reptiles, but in living land vertebrates the number has been 

 greatly reduced through loss and fusion. 



Skull of an Amphibian 



The descriptions are written for Necturus, but with slight 

 modifications may be applied to Cryptobranchus, an aquatic 

 amphibian somewhat similar to Necturus but much larger. 

 Examine both a dry skull and a specially prepared chondro- 

 cranium of Necturus. Necturus, like the fishes, has in reality 

 two skulls, a partially ossified chondrocranium on which is 

 superimposed a roof of membrane bone. Identify the foramen 

 mag?ni7n and the two occipital condyles. The fine lines at the 

 points of articulation between bones are known as sutures. The 

 upper jaw consists of three bones bearing teeth. Beginning 

 anteriorly these are the premaxillary, the vomer, and the palato- 

 pterygoid. The large, unpaired bone forming most of the floor 

 of the cranium is the parasphenoid. On the dorsal side, forming 

 the roof of the cranium, are the paired fro7itals, articulating 

 with the premaxillarles anteriorly, and posterior to these, the 

 parietals. On either side of the foramen magnum Is an exoccipi- 

 tal bone, each with Its occipital condyle for articulation with 

 the first vertebra. The otic capsules are partially ossified; the 

 opisthotic bones lie lateral to the exocclpltals and help form the 

 posterior corners of the skull, while the prootics are the small 

 bones in the anterior part of the otic region. Extending 

 obliquely forward from the opisthotic is the slender squainosal 

 bone. Beneath this and articulating with the lower jaw is the 

 quadrate. This bone represents an ossified remnant of the 

 pterygo-quadrate cartilage of the shark. The cartilage bones 



