526 
DE. A. GUNTHER’S DESCRIPTION OF CERATODUS. 
Before leaving the description of the skull, I have to add some notes on the cerebral 
cavity , with which the acoustic cavity is in direct communication. These parts may be 
seen in the vertical section of the skull (Plate XXXY. fig. 2). The brain itself was, un- 
fortunately, found to be destroyed ; but we suppose that, as in Lepidosiren, it is much 
smaller than, and does not fill, the cavity ; and it appears to be very similar in form to 
that of the other Dipnoi. We distinguish the broad canals for the diverging lobi olfac- 
torii ( ol a considerable excavation for the hemispheres, a small groove (the lowest por- 
tion of the cavity) for a well-developed pituitary gland ( pt ) which is still preserved, 
and the space for the corpus quadrigeminum and cerebellum, gradually passing into the 
medullary canal. 
Opposite to the origin of the nervus trigeminus, on each side of the brain-cavity, there 
is a large irregular opening leading into the cavity of the organ of hearing ; it is closed 
by membrane ( ac ), which is perforated by the acoustic nerve, and on which are distributed 
its branches. After the removal of the membrane, three irregular subdivisions of the 
cavity may be distinguished — one considerably larger than the others, and containing 
otolithic masses of a chalky appearance and easily dissolved in water. By microscopical 
examination they are found to be composed of the same prismatic crystals as in Lepi- 
dosiren (see Hyrtl, l. c. tab. 1. fig. 6). The three semicircular canals are rather wide ; 
the planes of the two inner ones are vertical and partly visible through the semitrans- 
parent upper surface of the cranial cartilage (see Plate XXXIV. fig 2.). The organ is 
entirely enclosed within the cartilage, without any other opening beside the communi- 
cation with the cerebral cavity. 
The hyoicl arch (Plate XXXV. fig. 1) is more complex than in Lepidosiren , and ap- 
proaches more the Teleostean type. It consists of a pair of ceratohyals, a basihyal*, 
and glossohyal*. As mentioned above, the ceratohyal (ch) is suspended from a 
tubercle at the base of the tympanic pedicle ; it is along, subcylindrical bone, externally 
well ossified, dilated at its proximal extremity into a subtriangular lamella. The basi- 
Iiyal (bh) is short, thick, cartilaginous, interposed between the ends of the ceratohyals 
and the acutely conical glossohyal (gh). 
The skeleton of the branchial apparatus does not differ from that of Teleosteous 
Fishes, but is entirely cartilaginous. There are five branchial arches, the last rudi- 
mentary and attached to the base of the fourth. There is no peculiar modification 
of any part of this apparatus ; and the middle pieces have the usual groove for the 
reception of the vessels and nerves. 
Vertebral Column (Plate XXX. fig. 2). 
Ceratodus agrees perfectly with Lepidosiren in the structure of the vertebral column. 
Its axis consists of a simple cartilaginous chorda dorsalis enclosed in a thick fibrous 
sheath, and with a gelatinous cylindrical thread along its centre. No transverse divisions 
in this notochord are visible ; and it passes uninterruptedly into the cartilaginous capsule 
* Absent in Lepidosiren. 
