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Some living fish that resemble their ancient ancestors, such as sharks, have multiple rows of dozens of heart valves, stored in a chamber called the conus arteriosus.

There is a smooth, thin-walled sinus venosus, a thin reticulate-walled auricle, produced laterally into two appendages, a thick-walled ventricle, and a conus arteriosus containing valves.

The conus arteriosus too begins to be divided into two chambers, and in Protopterus this division is complete.

The conus arteriosus becomes included in the ventricular cavity, but the sinus venosus still remains distinct, and its opening into the right ventricle is guarded by two valves which closely resemble the two venous valves in the auricle of the human embryo already referred to.

The sinus venosus is still a separate chamber, and the conus arteriosus, which may contain many or few valves, is usually divided into two by a spiral fold.