The Mechanisra of Pulmonary Respiration in Amphibians with Gill Clefts. 75 



The median choanal valve of Fischer corresponds to the choanal 

 valve of Necturus and is easily identified both in living and preser- 

 ved material. It is a narrow triangulär fold of mucous membrane 

 (Fig. 6), attacbed by one of its longer sides to the median margin 

 of the opening, by its short side to the anterior margin. The so- 

 called lateral valve of Fischer does not exist. 



The posterior choanal valve of Siren may be recognized in the 

 living animal by its movements, which are caused by a small muscle 



Fig. 7. 



Siren lacertina. Transverse section throagh the posterior part of the choanal muscle, pc. On the 

 median side of the muscle lies the cartilaginous trabecula cranii, from which many of the muscle 



fibres take their origin. 



acting on the antorbital cartilage (Fig. 8). The latter is a horizontal 

 plate of narrow triangulär form, whose broad end is directed caudad, 

 while its acute anterior end is embedded in the soft lip which forms 

 the lateral margin of the choana (Fig. 6). The posterior median 

 angle of this cartilage is attached by means of connective tissue 

 to a remnant of the trabecula cranii which emerges from the orbito- 

 sphenoid bone at the ventro-lateral angle of the brain case (Fig. 8). 

 The antorbital cartilage is moved by a striated muscle, m. posterior 

 choanae, which arises in part from the remnant of the trabecula 

 cranii, in part from the adjacent surface of the orbito-sphenoid bone. 



