GEOWTH OF THE TOOTH OF ECHIHUS. 
395 
direction is nearly horizontal, that is at right angles to the axis of the tooth, but not 
strictly so, as in their course from within towards the surface they point slightly 
downwards (see Plate VI. fig. 3, c). 
The tubes are cylindrical and unbranched: they extend from the surface to their 
inner limit in a nearly straight coui’se, and then divide all equidistant from the exterior : 
their divisions are almost uniformly dichotomous, the double tubes inosculating with 
the other channels of the less superficial portions of the tooth. The average diameter 
of these tubes is about inch. 
A good example of a transverse section of the enamel, magnified 400 diameters, is 
seen at Plate VIII. fig. 6, in which some of the tubes are sharply defined, a few con- 
taining air-bubbles, while others, more deeply seated, are looming hrdistinctly out of 
focus. The tubes appear to be open at their outer extremities. 
The portion of the tooth of Echinus which resembles in ultimate structure a section 
of molluscous shell, is the keel, as seen in transverse section (Plate VII. fig. 5). It is 
not precisely similar to any section in plane of the structure to which I have compared 
it, but it is like an obliquely transverse section proceeding from one part to another in 
an increasing curved obliquity. 
A transverse section of the columnar particles of such a shell as that of Pinna pre- 
sents an aggregation of polyhedral outlines, all of about the same aspect, whereas in the 
keel of the Echinus-tooth, as seen in similar section, the outlines of the cut prisms, 
which, close to the body of the tooth, are nearly equilateral pentagons or hexagons, 
become progressively elongated towards the enteric margin of the keel, so as to approach 
in form narrow oblongs with modified extremities. This depends, not on any alteration 
of the obliquity of the cut across the calcareous rods, but upon the change of their form 
towards their free ends, as wall be shown hereafter (see Woodcut III.). The prismatic 
sections look as if composed of cylinders so partially compressed, that the angles, where 
three or more lines meet, leave interspaces far larger than between the sides where the 
compression has apparently occurred. The similitude which these outlines have to the 
adaptation of compressible particles is of course only apparent, as the structure is com- 
posed entirely of hard calcareous rods ; but the change from true but small cylinders, 
which they are at first, to large rods, would involve, in their mutual adaptation, as they 
approached or came in contact, the same modification of form. 
It is the large interspaces, above spoken of, as seen in transverse section at the point 
where the outlines of some three or more prisms meet, which coincide with the rows of 
lacunse as seen in vertical section, and which determine their linear arrangement. Com- 
pare fig. 5, Plate VII. with figs. 7 & 8, Plate VIII. 
There is one other portion of the tooth peculiar and histologically characteristic ; that 
part of the body which faces the alimentary canal and the sides of the keel near the 
body of the tooth. The minute anatomy of this region, as seen in transverse section, is 
the nearest approach to the ordinary shell and sclerous echinoderm structure which any 
of the elements of the tooth exhibit. It consists of a loose open aggregation of tubes 
MDCCCLXI. 3 I 
