Mr. T. P. Barkas on Ctenoptychius. 353 
“The microscopical structure of this. tooth differs some- 
what from the preceding three varieties, but has more 
points of resemblance to C. pectinatus and C. denticulatus than 
to C. apicalis. It is composed of vascular and unvascular 
dentine. ‘The unvascular dentine covers the whole of the 
denticular surface of the crown, and the external surfaces of 
the coronal end of the base. The vascular dentine is con- 
tained within the layers of unvascular dentine, and is continued 
along the centre of the base nearly to its distal extremity, and 
tapers to a point in a direct ratio with the tail. The un- 
vascular dentine is a comparatively thick layer; and the sum 
of the thickness of the layers on the internal and’ external 
surfaces of the crown is nearly half the thickness of the crown 
itself, which is one twentieth of an inch ; it is permeated 
freely by fine calcigerous tubules, which spring from the 
vascular canals running along its internal surface and pass 
to the extreme periphery. 
“The tubules are rather fine, measuring from one five- 
thousandth to one ten-thousandth of an inch in diameter at 
their origin; in their course, which is vertical to the super- 
ficies, they are slightly wavy, branch freely but always dichoto- 
mously ; the branches are given off at an acute angle... . . 
“The structure of the crown of this tooth does not agree in 
any one particular with that of C. apicalis ; but there are many 
points of resemblance between it and the crowns of C. pecti- 
natus and C. denticulatus ; for they all have a layer of unvas- 
cular dentine covering the whole of the external surface, in 
which the tubules proceed to the extreme periphery; but 
the tubules differ m being fasciculate in their arrangement 
and larger in diameter in the two latter varieties, while they 
are not so in the former; and in all the medullary canals in the 
vascular dentine branch and anastomose, and the canals are 
dilated when branching or an anastomosis takes place ; 
they also run parallel with the axis of the tooth and 
with each other; but in C. wnilateralis the canals are consi- 
derably fewer in number, and give off a greater number of 
calcigerous tubules. 
‘“‘ I'he structure of the bases of these teeth cannot be com- 
pared; for the base of C. unilateralis is altogether unique in 
position and external characters, differing in this particular 
from the teeth of all other fishes, whether recent or extinct. 
“The arrangement of C. unilateralis in the jaw is not 
known.” 
Dr. Barkas, in a subsequent paper in the ‘ Monthly Review 
of Dental Surgery,’ named, described, and figured a new 
variety of Ctenoptychius under the name of C. aciculatus ; and 
