OUR BEST FEATURES 



cavity called the hyoid gill pouch. In the stur- 

 geon, a survivor of the primitive ganoids, W. K. 

 Parker's plates of a very young embryo show the 

 hyoid gill cleft lying in front of the upper part of 

 the hyomandibula, or upper segment of the second 

 gill arch. A spiracular cleft was also present in 



Olfacton/ 

 pit ^ 



Fig. 107. Embryo Sturgeon, Showing Gill Clefts 

 (after W. K. Parker). 



the oldest fossil lobe-finned ganoid Osteolepis 



(Watson). In the earliest known amphibians and 



reptiles the spiracular cleft may be represented 



in part by the otic notch (Figs. 17, 19) upon which 



the tympanic membrane was stretched. In the 



fishes the gill chamber behind and below the 



spiracle was covered externally by the bony 



opercular flap, but in the oldest known amphibians 



this bony gill cover has disappeared, leaving the 



prominent otic notch open behind. 



209 



