488 , MEMBRANE BONES. 



A larger series of persistent membrane bones are related to the 

 mandibular, and its palato-quadrate process. 



Overlying the palato-quadrate process are two rows of bones, one 

 row lying at the edge of the mouth, on the outer side of the pterygo- 

 palatine process, and the other set on the roof of the mouth super- 

 ficial to the ptery go -palatine process. 



The outer row is formed of the prsemaxilla, maxilla, jugal, 

 and very often quadrato-jugal. Of these bones the maxilla and 

 praimaxilla, as is more especially demonstrated by their ontogeny in 

 the Urodela, are partly derived from dentigerous plates and partly 

 from membrane plates outdde the moutli ; while the jugal, and qua- 

 drato-jugal when present, are entirely extra-oral. In the Amphibia 

 and Amniota the pra^maxillte and maxillae are the most important 

 bones in the facial region, and are quite independent of any cartila- 

 ginous substratum. 



The second row of bones is clearly constituted in the Dipnoi and 

 Amphibia by the vomer in front, then the palatine, and finally 

 the pterygoid behind. Of these bones the vomer is never related 

 -to a cartilaginous tract below, while the palatines and pterygoids 

 usvially are so. The position and growth of the three bones in many 

 Urodela (Axolotl) is especially striking (Hertwig. No. 442). In the 

 Axolotl they form a continuous series, the vomer and palatine being 

 covered by teeth, but the pterygoid being witiiout teeth. The vomer 

 and palatine originate from the united osseous plates of the bases of 

 the teeth, while the pterygoid is in the first instance continuous with 

 the palatine. 



In Teleostei, Amia, etc., there are dentigerous plates forming a 

 palatine and pterygoid, which in position, at any rate, closely cor- 

 respond with the similarly named bones in Amphibia; and there is 

 also a dentigerous vomer which may fairly be considered as equivalent 

 to that in Amphibia. 



In the Amniota the three bones found in Amphibia are always 

 present, but with a few exceptions am(mgst the Lacertilia and Ophi- 

 dia, are no longer dentigerous. The cartilaginous bars, which in the 

 lower tj^pes are placed below the palatine and pterygoid membrane 

 bones, are usually imperfectly or not at all developed. 



On Meckel's cartilage important membrane bones are almost always 

 grafted. On the outside and distal part of the cartilage a dentary is 

 usually developed, which may envelope and replace the cartilage to a 

 larger or smaller extent. Its oral edge is usually dentigerous. The 

 splenial membrane bone is the most important bone on the inner side 

 of Meckel's cartilage, but other elements known as the coronoid and 

 angular may also be added. In Mammalia the dentary is the only 

 element present (vide p. 485). 



On the roof of the mouth a median bone, the parasphenoid, is 

 very widely present in the Amphibia and Fishes, except the Elasmo- 

 branchii and Cyclostomata, and has no doubt the same phylogenetic 

 origin as the vomer and membranous palatines and pterygoids. 



