CERATODUS, PROTOPTERUS, AND CHIMERA. 499 
ing into the latter without a valve. The fact is that they form one cavity, not even 
divided by a constriction ; it is all the more necessary to insist upon this confluence of 
the two cavities, because in Lepidosiren and Protopterus a sinus is distinctly marked 
off from the auricle (fig. 9), though there are no proper valves between sinus and auricle. 
The form and extension of the basal fibro-cartilage, as we may call the body marked 
FC in fig. 4 in Ceratodus on the one hand, and Protopterus on the other, are correlated 
with the suppression and expression of the auricular antechamber. The general 
features of this remarkable body in Ceratodus are accurately described by Dr. Giinther, 
who also notes the fact that a corresponding structure exists in Protopterus. Owen, in 
his classical account of that fish, does not mention it. Hyrtl carefully described the 
basal fibro-cartilage of Lepidosiren, comparing its action, in relation to that of the 
hanging curtain valve or archway, with that of a piston and fan valve, whilst he sug- 
gested that it should be considered an imperfect septum ventriculi. There is not, I 
think, at the present time, any ground for regarding this structure as the forerunner 
of the septum ventriculi; but Hyrtl was apparently led to regard it as having the 
nature of a vertical cardiac “septum” from the fact that in Lepidosiren the auricular 
end of this mass gives attachment to muscular trabecule, which, according to that 
observer, incompletely divide the auricle into two chambers, a right and a left, of 
which the left receives only the pulmonary vein. 
Hyrtl gives no adequate figure of these parts, whilst both Bischoff and Owen are 
silent concerning it in Lepidosiren and Protopterus respectively. Dr. Giinther, in his 
description of the basal fibro-cartilage of Ceratodus, mentions the existence of one in 
Protopterus. 
In fig. 10 and fig. 11 the heart of Protopterus is represented in such a way that in 
the former the observer has the auricular termination of the cartilage (FC) facing him, 
whilst its ventricular origin is exhibited in the latter. The difference between the 
auricular termination of this structure in Protopterus on the one hand, and Ceratodus 
on the other (as exhibited in fig. 4), is of some interest. I have cited above Hyrtl’s 
observation, that in Lepidosiren the incomplete trabecular septum of the auricle is 
inserted into this fibro-cartilage, which he regards as an incomplete septum of the 
ventricle. Compare with this the condition in Protopterus. In Protopterus there is 
no indication of an auricular septum; the broad knob-like end of the basal fibro-carti- 
lage projects into the auricle, and there ends in a point, giving attachment inferiorly 
on each side to a membranous fold (fig. 10, Sav), the two folds thus forming a feebly 
developed valvular separation between sinus and auricle. In Lepidosiren, according to 
Hyrtl, the sinus opens separately into the right and the left divisions of the auricle 
without valves in either case. On the other hand, in Ceratodus, where, as above re- 
marked, there is not only no division of the auricle, but no separation between sinus 
and auricle, we find the knob-like swelling of the basal fibro-cartilage (fig. 4, FC) occu- 
pying the same position as in Protopterus (that is, projecting from beneath the auriculo- 
