DR. A. GUNTHER’S DESCRIPTION OF CERATODUS. 
519 
visible, but the dentine passes into the bone across the whole base of the tooth. It is 
not at all improbable that the pulp-cavity disappears altogether with age. 
In our specimens the structure of the bony base of the tooth differs in nothing from 
that of the remainder of the dentary bone (Plate XXXV. fig. 5) : there is the same 
spongious structure, the same proportion of bone-corpuscles, &c. 
Microscopical Structure of the Teeth (Plates XXXII. & XXXIII.). — The teeth of 
Ceratodus present that modification of the tubular structure which is known from Ces- 
tracion , Ptychodus , Psammodus, and other fossil genera*; the resemblance in this respect 
to the structure of a Psammodus tooth is particularly striking. In a vertical longitudinal 
section (Plate XXXII. fig. 2) the substance of the mandibulary tooth is seen to be tra- 
versed in the direction from the root towards its upper surface by about fifty-five or 
fifty-seven medullary canals, following a slightly undulated course, and running nearly 
parallel to, and at nearly equal distances from, one another. Some of them dichotomize ; 
but no anastomosis can be observed between them. Their terminations clearly correspond 
to the punctate impressions on the surface of the crown ; and those canals which do not 
actually terminate on the surface of the crown, have the ends surrounded by a great 
number of dentinal tubes ramifying in every direction. The boundaries between their 
respective systems are indicated by an intermediate space, into which the dentinal tubes 
do not penetrate, or in which only their minute terminations can be traced. 
In a horizontal section (Plate XXXIII. fig. 2) made near the crown of the tooth the 
lumina of the medullary canals appear opaque, of an irregularly ovate shape, surrounded 
by an opaque ring. The opaque centre and opaque ring are separated by a clear inter- 
space traversed by the wavy dentinal tubes. These tubes penetrate through the dark 
ring, branching off into a great number of finer and extremely minute tubules, so that 
the outer periphery of the ring appears to be surrounded by a crown of tubules like a 
fungoid growth. The width of the clear interspace is about equal to that of the dark 
ring. Sometimes the lumina of two or three and even more medullary canals are sur- 
rounded by the same dark ring ; they represent the branches of an originally single 
medullary canal which has been split up by bifurcation into two, three, or more branches. 
Each of these branches retains its own system of dentinal tubes, not anastomosing with 
that of the next. 
All these remarks refer to the principal vertical medullary canals in the body of the 
tooth ; but towards the surface of the processes (prongs) of the tooth (in the same hori- 
zontal section) smaller medullary canals may be observed which have a horizontal posi- 
tion, running obliquely towards the surface, and ramifying in an irregular manner into 
dentinal tubes which penetrate near to the enamel-like surface of the tooth. These 
canals are entirely similar to those figured by Peters in Protopterus (Mull. Arch. 1845, 
Taf. 3. fig. 4). There are no bone-corpuscles in any part of this section. 
In a second horizontal section, made near the base of the tooth (Plate XXXIII. fig. 3), 
the lumina of the medullary canals are rather wider than in the first ; they are open, 
* See Owen, £ Odontography,’ p. 11. 
4 C 
MDCCCLXXI. 
