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PACIFIC SCIENCE, Vol. XV, April 1961 
OTHER VEINS 
A large azygos vein is present just to the right 
of the midline; it is 1.25 cm. in diameter and 
terminates in the anterior vena cava. No left- 
sided azygos vein is present. 
An extradural intravertebral vein is present; 
at the level of the 12 th thoracic vertebra it lies 
to the left of the cauda equina. At this level it 
measures 1.75 cm. by 1.25 cm. It is somewhat 
larger at the level of the 3rd thoracic vertebra. 
The poor preservation of the specimen pre- 
vented detailed examination of the connexions 
of the vein. It definitely has connexions with the 
azygos vein and with the lumbar veins related 
to the stellate renal veins at the lower poles of 
the kidneys as in Phoca. These are clearly shown 
after the venous system had been injected with 
coloured gelatin. 
LUNGS 
Superficial examination (no casts were made) 
suggests that the arrangement of the bronchial 
tree is symmetrical as has been described in other 
seals (Brown, 1958). Histological examination 
reveals the presence of numerous features 
described by Pizey ( 1954) in the lung of Phoca. 
The cartilages of the tertiary bronchi are con- 
tinued far to the periphery of the lung, and 
bronchial mucous glands are frequent. The lung 
is divided into numerous lobules by well-marked 
septa of loose connective tissue. The bronchioles 
possess a series of myo-elastic valves which are 
similar to those of Phoca but not as marked as 
those in Tur slops (Wislocki, 1929). 
TESTIS 
Each testis weighs 1.9 g. and measures 25 X 
15 X 10 mm. in its greatest diameters. The sem- 
iniferous tubules are inactive, immature, devoid 
of lumina, and average 50/ /x in cross-sectioned 
diameter. The interstitial tissue is relatively 
abundant. Some of its cells are large, polyhedral, 
and heavily vacuolated, but the majority are 
small, fusiform, and have densely stained nuclei. 
The connective tissue between the seminiferous 
tubules is loosely arranged and oedematous. 
These appearances suggest that the gonad may 
well have been precociously enlarged at birth 
with hypertrophy of the interstitial tissue as has 
been described in Halichoerus, Phoca, and Mi- 
rounga (see Harrison, I960, for references). 
DISCUSSION 
It is difficult to come to any definite conclu- 
sions from the examination of this skeleton of 
Monachus schauinslandi , not only because of the 
lack of comparative material but also because 
of the extreme youth of the animal. It is obvious 
that it is more closely related to M. tropicalis 
than to M . monachus but this is to be expected 
because of its geographical position. Kenyon 
and Rice (1959) suggest that further study may 
indicate a closer relationship between the Lay- 
san and the West Indian monk seals and that 
M. schauinslandi may possibly be a race of M. 
tropicalis. 
The Laysan monk seal possesses certain vas- 
cular ( venous ) modifications seen in other Pin- 
nipedia. These are a duplicated posterior vena 
cava, a stellate renal plexus, a caval sphincter, 
a hepatic sinus, and an extradural intravertebral 
vein (Harrison and Tomlinson, 1956). There 
are, however, certain differences. The pattern of 
duplication of the posterior vena cava is more 
complicated than in any seal so far described. 
There is evidence of persistence of several 
anastomotic channels between the two limbs of 
the posterior vena cava. This could be inter- 
preted as persistence of an embryonic state in 
which primitive anastomoses have become en- 
larged rather than suppressed. The hepatic sinus 
is not as large as in Phoca or Mirounga , whereas 
the curious dilatation of the intrahepatic part of 
the vena cava is undescribed in seals. The sphinc- 
ter is not as large or as complete as in other 
Pinnipedia. It could be argued that the monk 
seal shows less vascular (venous) specialization 
than Phoca, Mirounga, Leptonychotes, Lobo- 
don, Halichoerus, and Hydrurga, but more than 
Zalophus. This could mean that monk seals are 
not able to dive for so long a period or as deep 
as these forms. No observations have been made 
on the diving abilities of the Laysan monk seal, 
but Kenyon and Rice (1959) note that these 
seals occur regularly only on islands having ex- 
tensive areas of shallow shoal water and that 
they appear to feed primarily on bottom-living 
fishes that they could obtain only in shallow 
water. They do travel over deep water, though 
not necessarily at any great depth. It must, how- 
ever, be emphasized that the specimen described 
here had coarctation of the aorta. We are not 
