THE HEART AND ARTERIES OF POLYODON 411 



Thus there are no trabeculae crossing the region opposite the 

 opening, but a crescent of them radiates around it, their con- 

 traction doubtless focusing the blood on this point. The weak- 

 est place in the wall, therefore, is opposite the mouth. The 

 sino-auricular valve is a pair of folds on the left placed oblique 

 to the sagittal plane with the ventral end of the slit between 

 them more medial than the dorsal. The slit makes an angle 

 of about 45 degrees with the perpendicular. The nearly round 

 auriculo-ventricular opening, is placed lateral, ventral, and an- 

 terior to the opening from the sinus. It is guarded by the auri- 

 culo-ventricular valve shown in fig. 14 B.^ 



The ventricular part of the heart is almost completely sur- 

 rounded and concealed by large lobes of lymphoid tissue appended 

 to its outer wall and richly supplied from the coronary artery. 

 Otherwise it presents no features calling for special mention. 



The conus arteriosus of Polyodon is well developed. The 

 number of valves, however, is relatively few as compared with 

 some other ganoids. Figs. 1 and 2 each represent a conus that 

 has been cut along the mid-ventral line and opened to show the 

 cusps of the valves. In fig. 11a portion of- the lateral wall has 

 been removed exposing the valves within. It will be seen 'from 

 these figures that in this species the conus is a variable structure, 

 at least in regard to the number of valves. Perhaps the most 

 common form has three valves of four cusps each, the cusps 

 being arranged in longitudinal rows as is usual among elasmo- 

 branchs and ganoids. Between the first and second valve there 

 is a considerable space. In some individuals this space is occu- 

 pied by another valve which may be either rudimentary (fig. 11), 

 or well developed (fig. 2). Although there are typically four 

 more or less equal cusps to each valve their number and form 

 vary and all stages from the merest rudiment to well developed 

 cusps can be found. In some cases, as in the second tier shown 

 in figure 1 , multiplicity seems to result from a division of one of 

 the cusps. In agreement with most other forms the first (most 

 anterior) valve, supposed to correspond with the single valve 



^The figures in this paper were made by ^Ir. Wm. T. Oliver from the original 

 drawings by the writer. 



