MESOPTERYGOID IN CERTAIN liEPTILIAN SKULLS. 455 



23. On the Persistence o£ the Mesopterygoid in certain 

 Reptilian Skulls. By R. Bkoom, F.R.S., C.M.Z.S. 



[Received April 4, 1922 : Read April 25, 1922.] 

 (Text-figures 5-7.) 



In 1878, Kitchen Parker described the structure and develop- 

 ment of the skull in the Common Lizard, and figured a section 

 of a fairly advanced Zootoca showing the region whei'e the ptftry- 

 goid articulates with the basisphenoid. A fairly large intercalary 

 cartilage is shown to be present, but Parker apparently did not 

 consider it to be of any important morphological value. 



In 1901, Howes and Swinnerton described the development 

 of the skull of Sphenodon punctatus (Trans. Zool. Soc. xvi. p. 1. 

 1901), and showed that there is also a cartilage similar to that 

 noticed by Parker in Zootoca, interposed between the pterygoid 

 and the pterygoid process of the basisphenoid. They compare it 

 to the meniscus found in the articulation of the jaws of most 

 mammals, and give to it the name meniscus pterygoideus. 



For many years I have been inclined to think that this little 

 cartilage would yet turn out to be of considerable morphological 

 importance, but until recently I have never been able to get any 

 clear light on it from either skeletogenesis or palaeontology. 

 Within the last few years, however, two papers have appeared 

 which seem to show with hardly any doubt that this little 

 meniscus p)terygoideus of Howes and Swinnerton is really a 

 persistent piscine mesopterygoid. 



Before discussing the nature of the element, it will be well to 

 consider in some detail its structure and relations, as, so far as I 

 am aware, it has never hitherto been at all fully described or 

 figured. 



Among living animals it is only known to occur in Lizards and 

 Sphenodon, and as it can be seen even better in the primitive 

 Lacertilian Agama than in Sphenodon, I will give a few figures 

 of it as it occurs in a larval Agama hispida. 



Text-fig. 5 represents a graphical reconstruction of the middle 

 part of the base of the skull of the embryo Agama with a skull 

 length of 7 mm.* The membi"ane bones of the skidl are all 

 ossified at this stage, but in the cartilage elements, such as the 

 quadrate, ossification has just started as an exostosis. As is seen 

 in the figvu-e given, the pterygoid is well ossified, and extends far 

 forward, though it does not meet the prevomer as in Sphenodon. 

 The basicranial cartilages are still unossified, and from the basi- 

 sphenoid region there passes outwards a well-developed basiptery- 

 goid process to support the pterygoid ; but it \\\S\ be seen that it 

 only supports the pterygoid indirectly, as a large independent 



* I am indebted to Prof. J. P. Hill for having kindlj- sectioned the Agama sluill 

 for me. 



