520 
DR. A. GUNTHER’S DESCRIPTION OF CERATODUS. 
not filled with opaque substance, and the opaque ring is narrower and very close to the 
periphery of the canal, with a very narrow clear interspace. On the level of this section 
fewer of the medullary canals are bifurcate. Bone ascends from the base of the tooth 
to the level of this section, especially at the ends of the tooth ; and some bone-corpuscles 
are observed also in the notches between the processes near the surface. 
In vertical (Plate XXXIII. figs. 4 & 5) and horizontal (Plate XXXIII. fig. 6) sections 
of a large tooth (2 inches long) of the fossil Ceratodus runcinatus I find exactly the same 
structure, making due allowance for the much greater age of the individual, which must 
have been very large, and for changes due to fossilization. We find the medullary canals 
arranged in the same manner, of nearly the same width, surrounded by a dark ring from 
which densely set (fungoid) dentinal tubes proceed. In consequence of greater attrition 
of the tooth’s surface, all the stems of the medullary canals penetrate to the surface, and 
the corresponding punctations lead directly into the canals. 
On the other hand, the microscopical structure of the tooth of Psammodus (Plate 
XXXII. figs. 6 & 7) does not sufficiently agree with that of Ceratodus to justify us in 
assuming, from this part alone, a close affinity between these two fishes. The medullary 
canals are conspicuously narrower, the dentinal tubes are much less numerous and more 
simple ; there is scarcely a trace of a dark ring ; and no tufts of surrounding minute 
tubules can be seen. 
The dentition of Lejpidosiren differs still more, as far as the minute structure is con- 
cerned. That of the upper tooth has been examined and figured by Owen (‘ Odonto- 
graphy,’ p. 168, pi. 59. fig. 4). The medullary canals form there a network; the main 
branches run “ nearly parallel with the plane of the upper surface of the tooth.” In 
the mandibulary tooth (which I have examined) (Plate XXXII. figs. 3, 4 & 5) the 
medullary canals are few in number, chiefly running from the base towards the upper 
parts of the tooth, emitting coarse processes which abruptly break up into very fine 
spreading ramifications. Near the anterior and posterior surfaces of the tooth the 
medullary canals are narrower, running in an oblique or vertical direction, parallel to 
the outer surface of the tooth, emitting dendritically branched processes. In a horizontal 
section of the tooth the lamina are not surrounded by a dark ring ; their arrangement 
and size, as well as the distribution of dentinal tubes, is exceedingly irregular. 
Before we proceed to the examination of the skeleton, it must he mentioned that the 
general arrangement of the muscles, especially of the trunk, is identical with that of 
Lepidosiren ; the limit between the dorsal and ventral series is indicated by the course 
of a continuous mucous canal corresponding to the lateral line. 
The Skeleton. 
The greater part of the skeleton is cartilaginous ; where ossification appears, it is in 
the form of a covering replacing the perichondrium and more or less completely enve- 
loping the cartilaginous substratum, but never taking its origin in the interior and by 
