IX.] THE COMMON FROG. 143 



there is no true piilmo7iary circulation — such an air-sac 

 (whether single or double) is termed a sivim-bladder, 

 and a structure of the kind is found in very many 

 fishes. The swim-bladder of ordinary fishes neither 

 receives blood directly from the heart by an artery 

 like the pulmonary artery of higher anim.als, nor does 

 it return blood directly to the heart. 



The transition, however, from a lung to a swim- 

 bladder is a graduated one. We have just seen that 

 in Proteus, though blood is returned from the lungs 

 direct to the heart, yet that not all the blood is so 

 returned. On the other hand in another animal, 

 Ceratodits, though blood is not brought to its air-sac 

 directly (which is therefore a swim-bladder and not a 

 lung), yet for all that blood is sent fro7n it direct to 

 the heart. 



Ceratodus (or as it is locally called '' flat-head ") is 

 a fish of Queensland, closely allied to Lepidosiren, 

 and is a very noteworthy animal apart from and in 

 addition to its peculiarly transitional structure as 

 regards its air-sac. 



It is, indeed, the last of an ancient race, a spe- 

 cies of the same genus (known almost exclusively 

 by its teeth), being found fossil in strata of oolitic 

 and triassic date. It was discovered by the Hon. 

 W. Foster, M.C.A. Mr. Gerard Krefift, F.L.S, 

 Curator and Secretary of the Sydney Museum, 

 first described and figured the animal in 1870^ and 

 at once correctly referred it to the genus Ceratodus, 

 which up to that time was supposed to be entirely 

 extinct. Its further determination was eff"ected by 

 * See Proc. Zool. Soc. 1870, p. 22. 



