MB. P. CHALMERS MITCHELL ON THE 



[June 16, 



presence or absence of the ambiens muscle as of primary import- 

 ance. He divided birds into the Homalogonatse, which possess the 

 muscle, and the Anomalogonatse, in which it is absent. Here and 

 there among groups which certainly must bo associated with the 

 Homalogonatous birds there are instances in which the ambiens 

 is absent, and in which Garrod believed the ambiens to have been 

 present, but recently lost. It is of great interest tlierefore to 



Fiff. 4. 



M.i. 



ptR.a. 



FllEX. COM. 

 Leg-muscles of Opisthocomus cristatus showing vestigial ambiens. 



Glut. 2, Attachment of gluteus medius. Gluti 3, Attachment of gluteus tertius. 

 Glut, ant., Gluteus anterior. II. 2. Flexor perforans et perforatus ihdicis. 

 III. 2, flexor perforans et perforatus medii. Pur. 2, Peroneus seeundus. 

 I. Flexor longus hallucis. II. Flexor perforatus indicis. III. Flexor 

 perforatus medii. IV. Flexor perforatus quarti. 



find a species different individuals of which show so great 

 variations in the condition of the ambiens muscle, reaching from 

 the normal complete condition found by Garrod to the extremely 

 vestigial condition in the specimen from which fig. 4 was drawn. 

 Some time ago, in a paper communicated to this Society ', I 

 recorded the discovery of vestiges of the ambiens in the case of two 



' " On the Perforated Flexor Muscles in some Birds," P. Z. S. 1894, p. 495. 



