520 ANATOMY OF VERTEBRATES. 



postcaval aperture, and a slight ridge indicates the remains of the 

 upper fold, forming the boundary of the ( sinus venosus.' In the 

 great Anteater I observed that the resemblance to the auricular 

 valve in Reptiles was rather closer : — the entry of the postcaval 

 was guarded as usual by the Eustachian valve, or homologue of 

 the lower of the two semilunar valves between the sinus and the 

 auricle in the Crocodile (vol. i. fig. 339): and here there was 

 also a narrower valvular fold or ridge on the opposite side of the 

 postcaval orifice, answering to the second valve (ib.) : a ridge is 

 continued from both valves toward the opening of the precaval. In 

 the Elephant, also, which shows its rodent affinity in the two pre- 

 cavals, there is, besides the ' Eustachian ' between the orifices of 

 the postcaval and left precaval, a remnant of the upper valve ex- 

 tending from the posterior side of the orifice of the right precaval. 



The inner surface of the right ventricle is smooth and even, 

 little broken by trabecular, in Rodents and other Lissencephala. 

 Two or three slender ' mammillary columns ' send tendinous 

 chords to the tricuspid valve in the Porcupine and Hare. The 

 apex of the heart is sub-bifid in the Hare and Acouchi : it is simple 

 and obtuse, with the ventricles broader and rather flattened from 

 before backward, in the Beaver : it is relatively longer and less 

 obtuse in the Water-vole : in neither of the aquatic Rodents are 

 the foramen ovale or ductus arteriosus kept patent. In most 

 Rodents the right ventricle reaches to the apex: in Helamys 

 it even descends lower than the left ventricle. The heart is short 

 and obtuse in the Sloths: the auricles almost cover the basal 

 part of the ventricles : the pericardium adheres to the diaphragm 

 by loose cellular tissue, and the thoracic part of the postcaval 

 is short. The pericardium is not so attached in the Armadillos, 

 and the heart is more oblong in shape, with the apex more 

 sinistrad : the lower third forming the apex is due wholly to 

 the left ventricle, from the basal part of which the right ven- 

 tricle projects, like an appendage, in Dasypus Peba. Orycteropus 

 has the Eustachian, but not the Thebesian, valve : the muscular 

 walls of the left ventricle are four times thicker than those of the 

 right ; but are almost smooth internally. With an unusual 

 thoracic convexity of the diaphragm, in the Mole, is associated a 

 less symmetrical position of the heart than in other Lissence- 

 phalans. 1 The tenuity of the pericardium is a characteristic of 

 many Insectivora : notably of the Hedgehog. 



C. Heart of Cetacea. — In these marine and fish-like Mammals 

 the heart, like the brain, shows higher characters than in the 

 preceding subclasses. The pericardium extends down upon 



1 cxxn'. torn. iv. p. 486. 



