CERATODUS, PROTOPTERUS, AND CHIMARA. AQT 
segment of the longitudinal (spiral) valve is seen projecting from the left-hand side of 
the divided wall of the lower part of the conus (compare fig. 7,Aandc). The wall 
of the lower part of the conus is continued downwards to form the arch-like auriculo- 
ventricular valve. The free depending portion has a perfectly smooth surface ; but this 
is not the case higher up. In fact the wall of this ower segment of the arterial cone 
is richly provided with peculiar pocket valves, which have not hitherto been described. 
These valves are so delicate that they might easily escape detection, or, in a specimen 
which was less excellently preserved than the one now examined, might be destroyed. 
An enlarged view of this portion of the wall of the arterial cone, pinned out flat and 
advantageously illuminated for the purpose of showing the valves in question, is given 
in fig. 5. It will be observed that there are three rows of these valves, of which the 
lower is the most complete, exhibiting as many as eight separate flaps, whilst these, 
and more especially the flaps of the middle series, exhibit a tendency to reduplication 
by transverse division. The three rows of valves correspond to three folds on the 
concave surface of the spiral longitudinal valve. The valves thus disposed consist of 
delicate membranous flaps (some as large again as, others smaller than, the area of a 
common pin’s head), which are fixed to the wall of the cone by their lower borders ; 
and most of them hang freely by the other three sides, except for the presence of more 
or less numerous chorde tendinew, which attach them loosely at various points (see 
fig. 6). Some of the larger valves are attached by the two sides right and left, as well 
as by the base, so as to form regular watch-pocket valves like those in the upper limb 
of the cone. 
The delicate and diminutive nature of some of these valves suggests very forcibly 
that we have here the remnant of an ancestral condition in which more powerful 
pocket valves were present throughout the extent of the conus, and that these have 
dwindled in proportion as the longitudinal spiral valve, characteristic of Dipnoi, has 
developed. That this is the case is further pointed to by the fact that in Ceratodus 
pocket valves are still strongly developed in the upper part of the conus, into 
which the longitudinal valve does not reach, whilst, on the other hand, in Lepidosiren 
and Protopterus, where the longitudinal valve does extend into the upper vertical limb 
of the conus (fig. 7B), no pocket valves are present in that region. 
Seeing that the delicate valves of the lower part of the conus of Ceratodus were 
such as to escape attention, it occurred to me as possible that similar delicate pocket 
valves might exist in the corresponding part of the conus of Protopterus; and accord- 
ingly I searched for them in that position and found them (fig. 12). 
Before leaving the heart of Ceratodus and passing on to that of Protopterus, there 
are three additional points which a comparison of my own notes with Dr. Giinther’s 
description leads me to consider as worthy of mention. First, as to the number and 
form of the pocket valves in the upper vertical segment of the cone in my specimen. 
Dr. Giinther examined the hearts of two specimens of Ceratodus; and, knowing the 
