THE NORTHERN RIBBON-FISH. 21 
of the pectoral arch, the large median urohyal element bounding it in front and 
below. The heart nearly fills the cavity. It is comparatively small, and shares in 
the extreme lateral compression which affects the whole fish. From base to apex it 
measures about one inch and an eighth. The sinus venosus, very small and thin- 
walled, lies entirely dorsal to the auricle, and receives, as usually, the ducts of Cuvier 
and the hepatic veins. Between the openings of the latter a slight membranous 
partition extends into the cavity. The sinus venosus opens into the auricle by an 
aperture guarded by a thin, transparent, membranous valve, consisting of two flaps. 
These flaps are lateral in position, and each is tied down to the auricle by two 
membranous cords, those on the right side reaching to the base of the auricle, the 
Fig. 9. Tae Heart (x 1). 
a., auricle ; c.a., conus arteriosus; d.c., ductus Cuvierii ; h.v., hepatic veins ; 
8.v., Sinus venosus ; v., ventricle. 
left-hand ones being attached near the valve. The cords are attached throughout 
their length, and apparently are the result of a strengthening fold accompanying 
the expansion of the internal lining which form the valve. The auricle is a flattened 
cone, lying over the ventricle and conus anteriorus. From each side a small auricular: 
appendix projects downwards to partially embrace the ventricle. 
A simple, membranous, two-flapped valve guards the auriculo-venticular aper- 
ture. Its two flaps are anterior and posterior. The ventricle is small, elongated, 
and rounded below. In front, very close to the auriculo-ventricular orifice, it passes 
directly into the conus arteriosus, by a two-flapped semilunar valve, whose two 
deep pockets are situated laterally. The bulbus arteriosus has a wide lumen, with 
extremely thin, almost membranous walls. 
S) (WWE IME AIR Ww, 
THE principal facts in regard to the skeleton that admit of being further summarized 
are as follows :-— 
1. The enormous orbit is correlated with the development of a sub-cranial crest 
or keel, less extensive, however, than in. Regalecus. 
2. The supra-occipital crest is small, though larger than in Regalecus, and is 
separated widely from the ex-occipital and from the foramen magnum by the 
immense epiotics. 
