HEAD-MUSCLES IN GALLUS AND OTHER SAUROPSIDA. 533 



in Chamaeleou it is fixed. It is probable that the movability 

 of the quadrate in the two former groups is a secondary 

 phenomenon and not a primitive Sauropsidan condition. 

 According to Broom it was initiated by loss of the quadrato- 

 jngal bone. 



The formation of the processus ascendens (or epipterygoid) 

 from the pterygoid process in Crocodilia, Rhyncocephalia, 

 and Lacertilia was probably a development subsequent to 

 fixation of the ptery go-quadrate. Its development is inti- 

 mately associated in the Crocodilia with the upgrowth of the 

 pterygoid muscle; in the Rhyncocephalia and Lacertilia 

 vera with the upgrowth of the external pterygoid muscle. It 

 is not developed in Birds, Chelonia, or Ophidia. The *' epi- 

 pterygoid" of Chelonia is merely the anterior end of the 

 pterygoid process; on the other haud, in Rhiptoglossa the 

 anterior end of the pterygoid process, which also becomes a 

 part of the cranial wall, contains the basal part of a processus 

 ascendens (Broom). 



Fiirbringer, from an analysis of the adult anatomy, homo- 

 logised the levator maxillee superioris of Selachians with 

 the spheno-pterygo-quadratus of Lacertilia and the orbito- 

 quadratus of Birds, and deduced the theory that the monimo- 

 stylic (with fixed quadrate) condition of Crocodilia and 

 Chelonia is a secondary one, and that the streptostylic (with 

 movable quadrate) condition found in Lacertilia, Ophidia, 

 and Birds is the primary one. The theory sketched above — 

 based on comparison of developmental and adult features — 

 separates the streptostylic condition into two forms : a primi- 

 tive streptostylic pterygo-quadrute (Birds), and a secondary 

 streptostylic quadrate (Lacertilia vera and Ophidia). The 

 original Sauropsidan stock thus probably possessed a mov- 

 able ptery go- quadrate and a fixed (membranous) palato- 

 pterygoid bar ; the former has been retained only by Birds, 

 the latter only by Chelonia, Crocodilia, and Rhyncocephalia. 



The lower part of the mandibular myotome in Callus passes 

 through three stages of development. At first, like the 

 adductor mandibulse of Scyllium, it forms a single muscle 



