Dll. A. GUNTHER’S DESCRIPTION OF CEllATODUS. 
525 
sented in figure 4 of the natural size, and magnified in figure 5, it is readily understood 
why the fossil teeth are always found isolated, without, or with scarcely any trace of, 
their osseous support. The bones are of a spongy consistency, with a dense network of 
wide medullary canals and cavities. The articulary especially has in its interior large 
cavities filled with marrow. The tooth can easily he broken off, its inner basal margin 
being anchylosed to the dentary by a narrow strip of bone only, and the outer resting on 
a layer of connective tissue which intervenes between the tooth and the articulary. The 
cells of the cartilage (fig. 5) are arranged in concentric rows, — the outer strata containing 
a greater number of cells, of an elongate ovate shape (fig. 7), whilst the cells of the central 
portion are fewer in number and more rounded. The articulary and dentary bones meet 
near the top of a low but strong coronoid process, and again at the symphysis, which is 
formed by fibrous tissue, and may easily be severed by the knife. The two articular 
concavities are separated by a narrow groove ; and this part consists entirely of cartilage. 
A wide oval foramen penetrates the cartilage in a vertical direction, opposite to and 
outwards of the second notch of the tooth (fig. 3, ar'). In front of the jaw the cartilage! 
is expanded into a slightly concave lamella (lower labial cartilage). 
The gill-cover apparatus (Plate XXXV. fig. 1) consists of the same bones as in Lepi- 
dosiren ; but being more developed, it assists in determining the two slender bones in 
that genus. The operculum* ( h ) is a flat subrhombic bone, fairly protecting the gill- 
cavity; its upper edge is inserted in a long groove of the tympanic cartilage behind 
the base of the suspensorium. To its lower edge is attached, by fibrous tissue, the long 
styliform suboperculum f (7f) terminating at a considerable distance from the mandibulary 
joint. A small moveable piece of cartilage is found inside of the articulary groove of 
the opercle (Plate XXXIV. figs. 2 & 3, Jr) ; it is a rudiment of a prce operculum. The 
gill-cover is adpressecl to the head by a singular broad muscle , which takes its origin from 
the lateral edge of the occipital cartilage, attaches itself to the upper margin of the 
opercle, and penetrates into the soft portions of the gill-cover, descending as far as the 
subopercle. It is divided into eight or nine fascicles by fibrous sheaths which run 
parallel with the muscular fibres. 
The basal% bone (Plate XXXIV. fig. 3, o, and Plate XXXV. fig. 2, o ) covers the 
greater part of the lower surface of the brain-capsule ; it is lance-head-shaped, broadest 
between the tympanic pedicles, tapering in front, and still more behind, filling out the 
entire space between the pterygo-palatines, and extending backwards far beyond the 
commencement of the vertebral canal, to the level of the third neural spine. It is a 
thin bone, except in the middle of its length, where large medullary cavities are im- 
bedded in its substance. 
* In Protopterus, “ Prasoperculum ” of Owen; “ Xiemendeckelstiick” of Peters. 
t In Protopterus, “ Branchiostegal” of Owen (Anat. Vert.) ; “ Kiemendeckelstuck, o'” of Peters. There are 
also two opercular pieces in Lepidosiren paradoxa ; but their shape and place of attachment to the skull are, 
according to Bischoee, widely different from the same in Ceratodus. 
X Basi-occipito-spheroidal hone of Owen in Lepidosiren., 
