340 P'Tof. Owen'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 mammillaref. 



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 MenopotneX and Menobranchus^ 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 sethmoid, and expand upon 

 the 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 5[ 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 

 the flat cornea is on a perfect level : there is a small spherical lens, and a 

 membranous sclerotic : there is no trace of the vascular body called, in osse- 

 ous Fishes, the choroid gland. The diameter of the eyeball is about one line 

 and a half: it has no special muscles, whence the absence of 3rd, 4th, and 6th 

 cerebral nerves. 



The organ of hearing consists of a large labyrinth excavated in a thick car- 

 tilaginous case, without other external communication than the foramina for 



• Tab. XXVII. fig. 4,/. t lb. fig. 4, g. J lb. fig. 5. 



§ lb. fig. 6. II lb. fig. 3 & 4, A A. f lb. fig. 4, •• 



