352 Messrs. Hancock & Atthey on Reptile- and Fish-Remains 
rarely present, or are perhaps frequently buried in the matrix. 
They are placed at some little distance from each other; and 
the small external teeth, like those of the upper jaw, frequently 
exhibit considerable irregularity, though on the whole they 
are placed apart at pretty regular intervals. ‘The above de- 
scription of the jaws applies to those of the usual size ; but we 
have a mandibular bone which, if complete, would be upwards 
of three inches long, and. a maxillary or two of corresponding 
dimensions. 
The laniary teeth are grooved at the base; and here the 
peripheral dentine is a little infolded or plicated; and in fine 
specimens the surface of the crown exhibits a thin film of 
enamel. Traces of enamel, too, are occasionally found on the 
small teeth; but they are most frequently without it, probably 
‘in consequence of erosion. 
One curious fact in connexion with the occurrence of this 
species is worth recording. Several of our specimens were 
found concealed within the stems of reed-like plants, which 
bear somewhat the appearance of calamites. A single indivi- 
dual occurred in each stem, nearly filling it. How they got 
into this position, whether accidentally or otherwise, it is im- 
possible to form an opinion; but as, out of a score of indivi- 
duals that have been found, four or five have been so placed, 
it would seem that something more than mere chance has had 
to do with it. 
Note.—It is apparently on fragments of the jaw-bones and 
on the teeth of Rhizodopsis sauroides that Prof. Owen has 
founded his Dittodus parallelus, Ganolodus Craggesii, Chara- 
codus confertus, and the Batrachian genus Grastrodus. The 
figure of Ditiodus parallelus (pl. 1) seems to us to repre- 
sent nothing more than a fragment of either a mandible or 
maxilla of this fish, with a few pairs of the teeth im juxta- 
position, the rest having been removed either before deposition 
or in making the section. 
When two teeth grow up close together, as we have seen is 
not unfrequently the case in this species, the peripheral den- 
tine of the two is often united at the base, and then we have a 
‘“‘twin-tooth”’ in all respects similar to those figured of this 
so-called Dittodus, and just as well entitled to be compared to 
the “ Siamese twins.”” We have now before us numerous sec- 
tions, many of which were made several years ago, demon- 
strating this fact; and in one or two instances there are even 
three or four teeth so united. 
That which is denominated “ osteo-dentine,”’ in the apical 
part of the pulp-cavity, is, we apprehend, a mere film of the 
