466 Mabel Bishop 



as far posteriorly as the shoulder girdles, where, it will be remembered, the 

 anlage of a median pair of fore-limbs and a median double shoulder 

 girdle. are present [Plate lY, Fig. 3, a and b]. 



If the eosmobiotic theory is tenable, the heart of Teras XX may be 

 expected to show a greater degree of doubling than does the heart of 

 the preceding Teras. An examination of Plate VII will support this 

 hypothesis, for it is at once obvious that there are present two separate 

 hearts, each supplying a complete normal set of arteries to each com- 

 ponent and connected one with the other only by a very short communi- 

 cating branch between the inner subclavians which pass to the anlage 

 of the median pair of fore-limbs. For the sake of clearness the arteries 

 have been separated somewhat more in the illustration than they are 

 in the specimen, and the sinus venosi and branches of the dorsal aorta 

 have been omitted. 



A description of a normal chelonian heart will suffice for the hearts- 

 of Teras XX, and for the sake of easy comparison I repeat here very 

 briefly the conditions found. The heart of a turtle has but three 

 chambers, one ventricle and two auricles. The ventricle is not divided 

 by a septum into two complete cavities as in the mammals studied, but 

 only partially so, hence there is a vital difference in the character of the 

 blood passing to and from the heart. But it is outside of the limits of 

 the present investigation to go into details of reptilian versus mammalian 

 circulation, since the difference would have no direct bearing upon the 

 eosmobiotic series. Owing to the smallness of Teras XX, it would have 

 been impossible to study the interior of the hearts without making serial 

 sections, and that seemed hardly necessary, for all the other circulatory 

 conditions being normal, it seems Mdiolly probable that the interior of 

 the hearts is also normal. The right auricle receives venous blood from 

 the body, the left the oxygenated blood from the lungs through the 

 pulmonary veins. The single ventricle lies posterior to the auricles 

 and is partially divided into a right and left portion, the right one being 

 known as the cavum pulmonale. The left portion is the larger of the 

 two and is subdivided into a cavum arteriosum on the left, and a cavum 

 venosum on the right. 



The pulmonary artery of each component arises as in normal from the 

 cavum pulmonale and almost at its beginning bifurcates into right and 

 left branches, which supply the right and left lung respectively. The right 

 branch passes dorsally to the other vessels arising from the right hand 

 portion of the ventricle and courses parallel to the right aorta for some 



