CEEATODUS, PEOTOPTEEUS, AND CHIM^EA. 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-cartUage, as we may call the body marked 

 FC in fig. 4 in Ceratodvs on the one hand, and Protopterus on the other, are correlated 

 with the suppression and expression of the aujicular 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 trabeculse, 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 aiiricle 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- 



