1. Blood pressure recordings were made above and below each of the three tiers of valves in the conus of Heterodontus portusjacksoni. Flow in the ventral aorta was recorded with an electromagnetic flow-meter.

2. It was shown that the lower conal valves close first and that this closure occurred before the end of ventricular systole.

3. The middle conal valves closed before the lower ones opened. The closure of both of these sets of valves was caused in part by the circumferential decrease due to conus systole, and in part by the back flow this engendered.

4. The upper conal valves closed as the middle ones opened at the end of conal systole.

5. Paralysing the conus with topical tricain rendered the lower and middle conal valves incompetent and caused the upper conal valves to be closed following ventricular relaxation with an increase in back flow.

6. The hypothesis is advanced that the conus serves to postpone the closure of the upper tier of valves until the negative intrapericardial pressure generated by ventricular ejection has decayed to a lower value.

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