Truncits Arteriosus. 201 



more or less modified in the different genera. In some cases 

 one or other of them seem almost to disappear ; in others they 

 appear to be fully developed, and show distinct fusion with the 

 top of the spiral fold. 



There is in Hyla aurea a modification of the spiral fold, which 

 will be recorded under the detailed account of that genus, and 

 which is interesting physiologically, in that it suggests a theory 

 of the blood flow which as yet does not appear to have been 

 considered. The solid objects are drawn as dissected, and the 

 sections are taken from consecutive series, the most diagram- 

 matic and typical being chosen. 



Hylidae. 



1. Hyla mirea. (Plate XXXVIL, Figs. 1, 2, 3, 4, 5, 6.) • 



The opening from the Ventricle to the Truncus Arteriosus 

 is on the right of the spiral fold, at the extreme proximal end. 

 It is guarded by two laterally placed semi-lunar valves, con- 

 nected by cordae tendinae, with the inner wall of the Pylan- 

 gium. 



At the anterior end of the spiral fold, guarding the opening 

 to the Pulmo-cutaneous arch, and on the left side of the fold, is 

 one very definite semilunar valve (v.), attached dorsally, and 

 free ventrally, and one small flap of membrane which may 

 be taken as a half valve. It lies to the right of the larger valve, 

 and is fused with it, and lies dorsal to the turned top of the 

 spiral valve, so as to be invisible in a dissection from the ven- 

 tral side. The spiral valve in Hyla is very strongly developed, 

 there being a very definite spiral twist. (Fig. 1.) The free 

 edges are thicker than the main part of the valve, and are 

 softly rounded. It is attached dorsally only, and is free along 

 the whole ventral border. The right hand end of the synan- 

 gial half-valve fuses with the spiral valve a short distance pos- 

 terior to the anterior end of its free edge. 



The papilla on which the Carotids open (Car.) is most distinct, 

 its edges overlapping the opening to the sj^stemic to a slight 

 extent. 



Developed in connection with the spiral valve is an inter- 

 locking apparatus, presumably functioning in conjunction with 



