42 ALLEN 
connective tissue layer (Text-figs. 1 and 2, and Pl. VI, EOE} 
C.T.), which is continuous with the corresponding layer of the 
auricle and where such union takes place, the layer becomes 
much thicker (see Text-fig. 1). The trabecule carne of the 
ventricle (Pl. VI, fig. 40; T.C.V.) resemble the trabecule of 
the auricle close to the outer connective tissue layer, the blood 
cavities being very small. In addition to the 2 layers of the 
auricle, the ventricle has a thick mascular layer (Pl. VI, fig. 
40; M.L.) between the trabeculae carne and the outer connec- 
tive tissue layer, containing no blood spaces. The central cav- 
ity of the ventricle (Pl. VI, figs. 39 and 40; C.C.V.), which 
runs close to dorsal wall, is much smaller than the central cav- 
ity of the auricle. The anterior end of this cavity is continuous 
with the posterior end of the bulbus arteriosus, which represents 
the well developed conus arteriosus (Pl. VI, fig. 40; C. Art.) of 
the Elasmobranchs and Ganoids. ‘The entrance into the conus 
arteriosus is guarded by 2 semz-lunar valves (Text-fig. 2 and 
Pl. VI, fig. 40; S.V.), dextrad and sinistrad in position and 
having their inner margins free. These valves are similar to 
the auriculo-ventricular valves, except that each valve has 2 
cephalic processes, which continue along the dorsal and ventral 
walls of the bulbus arteriosus. A reverse current, caused by a 
retraction of the elastic walls of the bulbus, would entirely close 
these valves, allowing no blood to return to the ventricle. 
Bulbus Arteriosus (Pls. 1, Il and VI, figs. 1, 12 and 4o, and 
Text-fig. 2; B. Art.).— After leaving the ventricle the bulbus 
rapidly increases in diameter and then gradually tapers down 
into the ventral aorta or branchial artery (Pls. I. and II, figs. 
1 and 12; V.Ao.), which gives off the paired afferent branch- 
ial vessels to the gills. The walls of the bulbus are quite thick 
and the internal layers are thrown into longztudinal ridges or 
folds (Pl. VI, fig. 40; L.F.). The bulbus is richly supplied 
with blood vessels, which will be described later on. 
Microscopical Structure of the Heart (Text-figs. 1 and 2).— 
A transverse section through one of the auriculo-ventricular 
valves (Text-fig. 1) shows us that the outer connective tissue 
layer is greatly thickened in the dorsal portion of the auricle and 
at the union with the same layer of the ventricle. As in other 
vertebrates the muscle fibers are striated and run in all direc- 
