400 
ME. S. J. A. SALTEE ON THE STEHCTIJEE AND 
wards and inwards to the enteric region : the lateral fibres converge on either side to 
join them (see Plate VII. fig. 3). The fibres originate in minute acicular threads, pass- 
ing inwards from the extreme edge of the plates. They increase rapidly in length and 
slowly in circumference. They remain cylindrical and even in diameter till about the 
x^th of an inch in length : they then commence enlarging gradually from the proxi- 
mal to the distal extremity, and this alteration in shape progresses till they are fully 
formed, when they assume an attenuated club-shape. Up to within about a fourth of 
theu’ distal extremity the transverse diameter of the fibres is (when thus fully developed) 
equal in each direction, about the -g^th of an inch ; they then enlarge vertically, their 
breadth remaining nearly the same. 
When seen edgewise the fibres do not appear to enlarge in the last quarter of their 
length : when seen sidewise they enlarge rapidly, forming a broad sword-shaped termi- 
nation. The end is sometimes bifid. See Plate VI. fig. 7, aioi; Plate VII. figs. 3 & 5. 
When the fibres have reached near to their maximum they become altered in form (as 
before mentioned), they cease to be rounded, and assume a polyhedral shape, so as to 
adapt themselves more closely — the number of the sides developed being variously 4, 5. 
or 6, the two latter most commonly. This is seen in transverse section of the keel, 
w'hich, though cutting the fibres themselves with obliquity on account of their slanting 
position in the keel, shows their superficial form with sufficient distinctness. In figure 
5, a, b, Plate VII., this may be observed, the altered form of the polyhedral sections of 
the fibres in passing outwards towards the extremity of the keel depending on this pro- 
gressive vertical enlargement. Thus it is that the appearances remind the observer of 
an obhque section of the shell of Pinna; but, as has been remarked, to produce the 
sectional elongation, it would be necessary, in the case of the mollusk shell, that the 
obliquity of the section should be progressively increased, which here is not the case — 
the altered form of the pentagons and hexagons in the keel of the Echinus-tooth depend- 
ing on the increased vertical growth of the fibres. This will be better understood by 
referring to the diagrammatic illustration of Woodcut III. 
Woodcut III.* 
By a fortunate fracture in a tooth, hardly sufficiently consolidated to admit of being 
* Diagram of a keel-fibre, indicating at a, h, c three planes of section, and at o', V, c' the resultant cut 
surfaces. 
