340 Prof. Owgw's Description of the Lepidosiren annectens. 
face of the brain behind the hemispheres is an elongated bilobed tract corre- 
sponding to the base of the third ventricle*, and posterior to this a single 
subspherical corpus mammillare1. 
The nerves given off from the brain are the olfactory; the optic, which 
arise close together from the mesial line, traversing the second basal mass; 
the fifth pair, which are of very large size; the auditory, and the eighth pair 
of nerves. 
The brain bears a closer resemblance to that of the Perennibranchiate Rep- 
tiles than to the brain of any fish which has yet been described. Figures of 
the brains of the Menopomet and MenobranchusK are added to the plate in 
order to illustrate this resemblance, which is very striking as regards the 
Menobranchus on account of the rudimental condition of its cerebellum. In 
the low development of this part of the brain, and in the large size of the 
pineal gland, the Lepidosiren deviates in a marked degree both from the 
osseous and cartilaginous fishes. 
The olfactory nerves|| are more than twice the size of the optic; they pass 
forwards through foramina in the cartilaginous zthmoid, and expand upon 
thie posterior surface of the nasal sacs, the pituitary membrane of which is 
disposed in two series of short transverse folds, about twenty in each, as 
before mentioned : these olfactory sacs have no communication with the buc- 
cal cavity. 
; The optic nerves arise close together, as in the Skate, from the mesial 
line traversing the second basal mass ; they do not decussate as in the osseous 
Fishes. They are remarkably small, in Correspondence with the feebly- 
developed organs of vision. Each eyeball adheres to the skin, with which 
t level: there is a small Spherical lens, and a 
membranous sclerotic : there 
* Tas. XXVII. fig. 4, f. 
$ Ib. fig. 6. t Tb. fig. 4, 9. t Ib. fig. 5. 
