DR. A. GUNTHER'S DESCRIPTION OF CEEATODUS. 
563 
c. Origin of central medullary canals, which ramify in an irregular manner, 
chiefly in the direction towards the point of the prong. 
d. Narrower superficial medullary canals, running in an oblique or ver- 
tical direction, and emitting horizontal branches. 
Fig. 4. Horizontal section through the lower tooth of Protopterus, of natural size, the 
darkened part being that of which a magnified view is given in figure 5. 
Fig. 5. Part of a horizontal section through the front prong of the mandibulary tooth 
of Protopterus, magn. 216. It comprises several of the smaller medul- 
lary canals represented in fig. 3, d. Two are nearly horizontal, so that their 
whole extent is visible ; others run in a vertical direction. They ramify in a 
very irregular manner, and are not surrounded by a dark ring of calcige- 
rous tubes. 
Fig. 6. Vertical section through the tooth of Psammodus , magn. 216. The hori- 
zontal white line indicates that a portion of the depth of the tooth is left out. 
The upper portion contains three medullary canals, two penetrating to the 
surface, where they form the “ pores in the lower portion the canals have a 
more oblique direction, and consequently the vertical section passes slant- 
ingly through them ; the concentric lamellated arrangement of the dentinal 
substance round the canals is also conspicuous, whilst it has disappeared 
nearer to the surface of the tooth. 
Fig. 7. Horizontal section through the tooth of Psammodus, magn. 216, made nearer to 
the root than to the surface. 
PLATE XXXIII. 
Microscopical structure of the teeth (continued). 
Fig. 1. Vertical section through the medullary canals of Ceratodus forsteri, magn. 216. 
The clear horizontal line indicates that a portion of the length of the tooth 
is left out. 
a. Termination of a medullary canal in the pulp-cavity. 
h. Termination of a medullary canal in one of the punctate impressions 
on the surface of the tooth. 
Fig. 2. Horizontal section through the same tooth, near the surface of the crown, 
magn. 216. 
Fig. 3. Horizontal section through the same tooth, near its base. 
Fig. 4. Portion of a vertical section through a tooth of Ceratodus runcinatus (magn. 20), 
for comparison with the recent species (Plate XXXII. fig. 2). 
Fig. 5. Part of the same section (magn. 216), for comparison with the structure 
of the same part in the recent species (fig. 1). Three medullary canals 
open on the surface of the crown, producing the appearance of punctate im- 
pressions. 
Fig. 6. Horizontal section through a tooth of Ceratodus runcinatus (magn. 216), for 
comparison with fig. 2. 
