Triviicus Arteriosus. 205 



to be less curved than is usiiallv met with in the other Austral- 

 ian frogs so far investigated. 



The carotid papilla is present, l)ut does not appear to be 

 exceptionally large. 



SU^IIVIARY OF RESULTS. 



The Truncus Arteriosus is divided into two distinct portions 

 — the proximal Pylangium, between which and the Ventricle 

 are the Pylangial valves, and the distal portion or Synangiuni, 

 separated from the Pylangium also by valves. It is chiefly 

 with these valves and associated structures that this paper deals. 

 Various authors (see Table) have worked out these valves 

 in Rana, and the result has been such that a marked discre- 

 pancy is apparent. Also there appears to be some doubt as to 

 the relative position of the Synangial valves and the Pulmo- 

 cutaneous opening. Some authorities depict the Pulmo- 

 cutaneous opening in the Synangium, and the cups of the valves 

 facing anteriorly ; some place it in the Pylangium, with the cups 

 of the valves facing posteriorly. As these two positions would 

 necessarily give totally different methods of the blood flow 

 through the Truncus, it is of importance to ascertain the posi- 

 tive position of these parts. 



In the Australian frogs worked there is a constant number 

 of Pylangial valves (2), and also of Synangial valves (2), 

 the structure and development in the different genera 

 only being subject to variation. In every individual investi- 

 gated, the Pulmo-cutaneous arch has opened from the Pylan- 

 gium, and has been guarded by the Sjmangial valves which face 

 posteriorly, towards the cavity of the Pylangium. This position 

 of the Pulmo-cutaneous arch in relation to the Syangial valves, 

 together w^ith the interlocking structure, suggests the above- 

 mentioned theory of the blood flow through the truncus. 



In the following table the number and importance of Pylan- 

 gial and Synangial valves is indicated, together with the position 

 of the Pulmo-cutaneous aperture in the specimens examined. 



