Hawaiian Monk Seal — King and Harrison 
to the cotyloid notch that is not present in 
M. monachus or M. tropicalis (Fig. 4a) . 
FEMUR, TIBIA, FIBULA: These bones are not 
sufficiently well formed to be used for any 
comparison. 
PES: The previous description of the pes of 
M. monachus applies also to the pes of the 
present animal. There is practically no indica- 
tion of the insertion of the small claws on the 
terminal phalanges (Fig. 4 d) . 
VERTEBRAL COLUMN: Vertebral formula: 
cervical 7, thoracic 15, lumbar 5, sacral 3, caudal 
13. 
Cervical vertebrae. These are more like M. 
tropicalis than M. monachus in that the trans- 
verse processes of vertebrae 4-6 are divided into 
two branches, though the division is less pro- 
nounced than in the adult M. tropicalis. There 
is a general similarity to M. monachus, though 
this is probably more because of the similarity 
in age. The neural arches have completely fused. 
Thoracic vertebrae. These are similar to those 
of the young M. monachus except that the neural 
arches are narrower and do not lean so far 
posteriorly. 
Lumbar and caudal vertebrae. These are as 
previously described (King, 1956). 
RIBS: The articulation of the ribs is similar 
to that in the other monk seals. In order to in- 
ject the vascular system and remove the soft parts 
the cartilaginous parts of the ribs were cut away. 
ABDOMINAL VEINS 
The abdominal veins are thin and easily dis- 
tensible. The posterior vena cava is duplicated 
as in Phoca but displays a complicated arrange- 
ment of large anastomotic channels (Fig. 5 ) not 
hitherto described in other Pinnipedia. The 
right limb of the posterior vena cava is the 
larger, is almost straight, and lies a little to the 
right of the midline. The smaller left limb ex- 
tends from the pelvis on the left, passing some- 
what obliquely cranialwards to the right to join 
the right limb near the upper pole of the left 
kidney. A large anastomotic channel passes from 
the right limb at the level of the lower pole of 
the right kidney obliquely across the midline 
to join the left limb at the level of the middle 
of the left kidney. A smaller channel arises from 
the right cranial end of the anastomosis just 
289 
described, passes dorsal to the right limb, and 
enters it on the right at the level of the upper 
pole of the right kidney. Each limb and the two 
anastomotic channels receive numerous tribu- 
taries draining the renal stellate plexus as well 
as many vessels from the lumbar and pelvic 
venous plexuses. The right limb of the posterior 
vena cava is 2.5 cm. in diameter where it is 
formed by union of the iliac, lumbar, and most 
caudal renal tributaries. It is 3.0 cm. in diameter 
where it is joined by the left limb. The common 
trunk is 3.0 cm. in diameter throughout the 
8.0 cm. of its extent to the point where it is 
enclosed by hepatic tissue. 
The common trunk of the posterior vena cava 
is enlarged considerably where it enters the sub- 
stance of one lobe of the multilobed liver. It has 
the form of a dilated tube, 15 cm. in length and 
6 cm. in diameter in its cranial portion, lying 
on the ventral surface of the liver and sur- 
rounded on three sides by a narrow strip of 
hepatic tissue. Several large orifices of hepatic 
veins are present on the lateral walls of this 
dilated part of the posterior vena cava. Cranially 
this part of the posterior vena cava enters a 
hepatic sinus, nearly spherical in shape and ap- 
proximately 10 cm. in diameter. The sinus is 
partially surrounded by hepatic tissue, but in 
regions only a thin translucent wall covered by 
peritoneum separates it from the diaphragm. 
The sinus is divided by two narrow septa arising 
from its right wall. Six large hepatic veins open 
into the sinus. The capacity of the sinus is 
estimated to be 450 cc. Figure 6 shows the ap- 
pearances of the dilated vena cava and the he- 
patic sinus. 
The intrathoracic part of the posterior vena 
cava is 5 cm. in length and 3.5 cm. in diameter. 
No pericardial plexuses of vessels, such as are 
found in Phoca, were present and no veins 
drained into this part of the vena cava. An in- 
complete sphincter of striated muscle encircles 
the vena cava just cranial to the diaphragm. The 
dorsal part of the sphincter was 3.5 cm. high 
and was closely adherent to the vena cava; it was 
separated from the diaphragmatic muscle by a 
narrow strip of connective tissue. The fibres of 
the sphincter only partially encircle the vena 
cava so that on its ventral aspect the sphincter 
is narrowed to a bundle of closely packed fibres 
