536 
DE. A. GUNTHEE’S DESCEIPTION OF CEEATODUS. 
phragm to the middle of the pericardium, as it has been described by Hyrtl in 
Lepidosiren paradoxa , and as I have found it also in Protopterus , is not developed in 
Ceratodus. 
After the pericardium is opened (fig. 1) the ventricle ( v ) is observed on the right side, 
and the large single atrium (a) on the left. From the former arises a conus arteriosus * (b), 
forming a short spiral ; that part of it which is visible without further preparation, runs 
transversely from the right towards the left. Its muscular layer is much more developed 
along the lower side of the spiral than along the upper. The grooves between the divi- 
sions of the heart are covered with a thick layer of a dark semifluid fat. At the upper 
end of the conus the insertion of the pericardial sheath causes a slight swelling round 
this portion. The heart hangs freely in the pericardium, except at the two opposite 
poles, viz. where the sinus communicates with the auricle, and where the conus arte- 
riosus passes into the aorta. A single very short and thin filament fixes the point of the 
heart to the pericardium. 
We begin with opening the ventricle on the side facing the observer (figs. 2 & 3). 
Its parietes are thick, with well-developed trabecul® carne® on its inner surface. Its 
cavity is spacious, and incompletely divided into two by a papillary muscle (m) rising 
from the apical portion of the ventricle, and passing into a cartilage ( m !) which is con- 
siderably broader and thicker than the muscle ; a number of chord® tendine® fix the end 
of this body to the walls of the atrium, into the cavity of which the cartilage extends. 
The cartilage, with its muscle, lies in the plane of the atrio-ventricular opening, which is 
partially closed by it during the systole. Opposite to the base of this papillary muscle 
(which is also present in Lepidosiren and Protopterus , and has justly been described by 
Hyrtl as an incomplete septum ventriculi) is a broad valve ( c ) with semilunar margin; 
it likewise assists in preventing the blood of the ventricle from regurgitating into the 
atrium during the systole. 
The cavity of the atrium (fig. 3) is about twice as spacious as that of the ventricle ; 
it does not extend beyond the left half of the heart ; and there is no long, produced auri- 
cular process ; its walls are membranous, with a great number of thin muscular fasciculi 
decussating and forming an open network. Those of these little muscles which are 
connected with the cartilage are strongest, and terminate in short chord® tendine® ; but 
although a careful preparation under water gave a clear view of their arrangement, I 
could not convince myself that they indicated a division of the atrium, such as Hyrtl 
found in Lepidosiren par adoxa. 
The vessels carrying the blood to the atrium open into a sinus venosus which lies 
within the pericardium. The sinus communicates with the atrium by a simple ostium 
* Before I became acquainted with a paper by Gegexbaur in Jena. Zeitschr. f. Med. und Eaturwiss. ii. pp. 365- 
375, 1 distinguished the dilatation of the ventricle in Ganoids and Selachians under the name of Bulbus arte- 
riosus, from the swelling of the aorta of Teleosteans, for which I retained the term Bulbus aortce. But finding 
that Professor Gegestbauk had already proposed the term “conus arteriosus” for the former division of the 
heart, I shall, of course, adopt the prior nomenclature. 
