THE HEART AND ARTERIES OF POLYODON 451 



mouth of the pyloric gland it bifurcates. The anterior division 

 branches palmately into a number of arteries to the stomach, 

 a left posterior hepatic artery, which, as previously stated, anas- 

 tomoses with the anterior hepatics, and a large artery to the 

 anterior part of the pyloric gland. The posterior division breaks 

 up into three branches, one of which supplies the posterior part 

 of the pyloric caeca, one to the small intestine and one to the 

 side of the stomach. 



As is the case with many other forms the details of distri- 

 bution vary greatly with all of these arteries, and a wide latitude 

 must be allowed for individual differences. 



CONCLUSION 



The foregoing account of the general anatomy of the arterial 

 system of Polyodon shows that, while in many respects the 

 ganoid type, especially as exemplified by Acipenser, is quite 

 clearly indicated, there are, nevertheless, some features which 

 suggest elasmobranch and teleostean conditions. Among the 

 former may be mentioned the posterior coronary arteries which 

 to some extent resemble those of the skates, and among the 

 latter the orbitonasal artery of teleosts, although it must be 

 confessed that the homologies of this vessel are by no means 

 clearly established. Marked peculiarities of Polyodon, appar- 

 ently not common to other fish so far as known, are the absence 

 of anterior coronary arteries, the origin of the posterior coronary 

 in the epibranchial region, and the existence of anterior hepatic 

 arteries. The evidence of these characters, so far as it goes, 

 indicates that Polyodon is not very closely related to any of 

 the other forms. Whether its resemblance to the skates, both 

 here and in some of its skeletal structures, is anything more 

 than a chance parallelism may well be doubted. 



There is a close agreement between the account given here 

 and that of Allis's paper wherever the two overlap. Small 

 differences in regard to number and origin of vessels fall easily 

 within the limits of individual variation which, in Polyodon, is 

 very great. 



