HEAD-MUSCLES IN GALLUS AND OTHER SAUROPSIDA. 549 



be derived from tliat seen in the embryo of Chelone (i. e. a 

 single muscle with two heads^ one from the pterygoid process 

 of the quadrate, and one from the hind end of the palato- 

 pterygoid bar), and which undergoes very little modijScation 

 in subsequent development, but not from that found in 

 Rhyncocephalia and Lacertilia vera (where the external 

 pterygoid muscle grows up outside the pterygoid process to 

 the side of the skull), nor from that found in Crocodilia 

 (where au undivided pterygoid muscle grows up outside the 

 pterygoid process to the side of the skull.) 



The lingual muscles of Birds, in that they are only deriva- 

 ble from a genio-glossus and hyoglossus, which were attached 

 to the basihyobranchial and did not end fx-ee in the tongue, 

 are more like those of Chelonia and Crocodilia than any other 

 Reptilian group. 



These features of resemblance suggest at first sight a very 

 distant Chelonian relationship for Birds, but are in reality 

 only ancestral traits, which are also present in embryonic 

 stages of other Sauropsidan groups. The secondary fixation 

 of the pterygo-quadrate and atrophy of the elevator of the 

 pterygoid process, which occur in Chelonia, are strongly 

 marked differences from Birds. 



The Crocodilia resemble the Chelonia in the atrophy of the 

 upper part of the mandibular myotome and in the fixation of 

 the pterygo-quadrate, but these features have come about 

 in different ways in the two groups. They also resemble one 

 another in the condition of the lingual muscles. 



The Rhyncocephalia have preserved two features more 

 archaic than are found in any other Sauropsidan group — the 

 continuity of the ceratohyal, and the condition of the branchio- 

 hyoid muscle — but in the upgrowth of the external pterygoid 

 muscle and the condition of the lingual muscles are less 

 primitive than the Chelonia. Like the Chelonia and Croco- 

 dilia, they have preserved a fixed pterygoid bone. 



The Lacertilia vera present great resemblances to the 

 Rhyncocephalia in the condition of the external pterygoid 

 muscle and the linorual muscles. The chief differences are 



