186 Mr. T. Stock on the Genus Tristychius. 
Outicular Appendages.—Dimly seen ; but there are traces 
of them, and these do not differ (so far as can be made out) 
from those described as belonging to the last specimen. They 
are most visible on the extreme posterior edges of the halves 
of the nodule. 
Teeth.—On opening this nodule I thought I had settled the 
long-standing controversy as to the true nature of the bodies 
described by Agassiz as Ctenoptychius pectinatus, and believed 
by him to be teeth, but by Messrs. Hancock and Atthey* to be 
appendages of the dermis. A number of broad, low, suleated 
teeth were seen lying in much disorder, yet compacted toge- 
ther, and forming evidently no inconsiderable portion of the 
armature of a Selachian mouth. An examination with the lens, 
however, resolved the majority of these teeth into forms with 
which I had been made very familiar by the material previ- 
ously considered in this paper; and I must confess to a 
moment of keen disappointment as this fact became apparent. 
Nevertheless, after repeated examination of the specimens 
under every optical condition, and after a consideration of 
other evidence, I have gradually been led to adopt the view 
that Ctenoptychius pectinatus is a part of the dentition of 
Tristychius arcuatus,—that is to say, that the teeth described 
in the last specimen and well seen in this (teeth with ele- 
vated median cusps and well-developed lateral cusps, strongly 
grooved, and with short striated and rooted bases) are accom- 
panied in the same mouth, but in a different part of it, by teeth 
with no specially raised median or lateral elevations (the fasci- 
culations of these teeth may probably, however, be homologous 
with the cusps of the other kind), with a low, nearly straight 
area, with well-developed bases, fringed (in many specimens) 
by root-like attachments. The gradations between the two 
kinds (if we may hope that a clue has been given) is not so very 
surprising. Slightly elevate the grooved and (already) fasci- 
culated denticles of Ctenoptychius pectinatus, first at the middle 
and then at the sides, and you have Tristychius arcuatus 
teeth. Depress (and in some teeth the elevation is slight) 
the cusps of the latter, and you have the former. It is a great 
pity that the evidence furmished by the specimen is no clearer ; 
yet I consider it suflicient to warrant the expression of these 
views. In most of the teeth that are visible the distances 
between the cusps are much reduced when compared with 
some of those described in the preceding specimen. In one 
* Northumb. & Durham Nat. Hist. Trans. vol. ili. p. 116. See also a 
note by myself in the Ann. & Mag. Nat. Hist., April 1882, p. 256. 
Messrs. Hancock and Atthey acknowledged their great resemblance to 
teeth, 
