62 Proceedings of Royal Society of Edinhurgli. [sess. 
both of the sinus venosus and appendix. Four pulmonary veins 
without valves opened into it. 
The left ventricle formed the left half of the ventricular division 
of the heart both dorsally and ventrally. Its walls varied in thick- 
ness from IJ to inches, and it possessed strong trabeculae carneae. 
The papillary muscles were for the most part thick and stunted, and 
resembled mound-like elevations rather than papillae, and their 
chordae tendineae were very strong. The left auriculo-ventricular 
valve was not bicuspid, but consisted of four distinct triangular 
cusps continuous with each other at their bases around the auriculo- 
ventricular opening, and in this respect it resembled generally the 
heart of Sowerby’s whale described by me some years ago.* The 
four cusps were not uniform in size : one {a), the largest, was 
situated at the dorsal (superior) border of the opening ; one (h) at 
the dorsal, another (c) at the ventral (inferior) border of the left 
boundary of the opening ; whilst the fourth {d) was placed between 
c and the large dorsal cusp (a). A stunted papillary muscle arose 
opposite the interval between c and d ; another was opposite the 
interval between a and d ; a third lay between h and c, and in each 
case they distributed their chordae tendineae to the cusps between 
which they were placed. The chordae tendineae that passed to the 
adjoining borders of a and h did not arise from a thick stunted 
muscle, but from several slender elongated papillae situated between 
their borders. Four short but powerful trabeculae carneae passed 
from the septal wall of the ventricle across the cavity to the adjoin- 
ing part of the ventral wall, and would, doubtless, act as muscular 
moderator bands. Between these trabeculae and the dorsal cusp of 
the left auriculo-ventricular valve, an elongated and comparatively 
narrow passage directed the blood into the mouth of the aorta, 
which was guarded by its semilunar valves. 
The three great arteries which arose from the transverse part of 
the arch of the aorta had been cut across close to their origin. But 
if I may interpret the arrangement from the dissection which I made 
and figured many years ago in the heart of Balcsnoptera sihhaldii, t 
they were in all probability a right brachio-cephalic, a left carotid, 
and a left subclavian, from the last named of which a left posterior 
* Jour, of Anat and Phys.^ Oct. 1885. 
+ Trans. Boy. Soc. Edin., 1870, vol. xxvi. 
