AND ON THE CLASSIFICATION OF FISH. 551 
The nervus vagus in Lepisosteus passes out through a foramen 
in the lateral occipital bone, in Polypterus between the occipital 
and the mastoid. In Polypterus the longer right swimming-blad- 
der receives twigs from the right and left ramus intestinalis, the 
small left swimming-bladder from the left ramus intestinalis 
only. Lepisosteus has a single lateral nerve, Polypterus has two 
from the vagus—one upper and one lower ; the former runs near 
the upper mesial line beneath the scaly membrane, the lower 
one proceeds to the lateral line with the lateral lymphatic duct ; 
they both lie above the button-shaped extremity of the rib-like 
fleshy bundles, which must however be distinguished from the 
ribs, which are connected with the scales of the lateral line. 
Behind the vagus in Polypterus two other nerves leave the 
skull through foramina in the occipital bone, viz. the hypoglos- 
sus for the sterno-hyoid muscle and a nerve for the pectoral fin ; 
the latter also receives two spinal nerves. In Lepisosteus osseus 
four other nerves posterior to the vagus pass through the occi- 
pital bone; three, the first of which is very delicate, through 
foramina in the lateral occipital, the fourth through an aperture 
im the ascending portion of the basilar occipital bone. The two 
former unite externally with the nervus hypoglossus to supply 
the sterno-hyoideus. The two posterior ones go to the pectoral 
fins. Hence it is ciearly seen that no value can be placed on 
the number of the latter cerebral nerves or cranial apertures 
situated posterior to the vagus; and that a uniform number 
of cerebral nerves must not be sought for in the Vertebrata. 
The sympathetic nerve in the Ganoids resembles that of the 
osseous fishes: in Polypterus it runs on each side of the aorta, 
and is connected with the spinal nerves by very long communi- 
cating branches. 
We have still to mention a very remarkable appearance in the 
lower spines (of the caudal portion of the vertebral column) of 
the Ganoids; in Polypterus and Lepisosteus they remain as se- 
parate bones appended to the vertebral column, exactly as the 
lower spines of the tail of some mammalia*. The remarkable 
circumstance is, that these inferior spines in those Ganoids 
which have an osseous skeleton (Ganoidei holostei), do not appear 
to arise, as in other fishes, from the union of the lower apo- 
physes of the vertebral body (which in fishes in the early state 
are distinct osseous pieces), but that in Lepisosteus they are 
* Vergleich. Osteol. der Myxin., 97. 
