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broader surfaces. Trochite, with a circular hole, may be divided into 
the following species : 
The first species has striae passing nearly straight, from the centre 
to the circumferenee. This trochites, or vertebra, is figured and de- 
scribed by Rosinus, Tab. VII. Cap. III. Class A. 1, 2, 3,* and is here 
represented Plate XIII. Fig. 13. The stria; of this trochites appear, 
to the naked eye, to pass straight and undivided to the circumference 
but it may be discovered, by the aid of a magnifying glass, that the 
rays are not arianged exactly as might be expected, the same striffi 
not being continued all the way, but, in some places, apparently rami- 
fying; and in others, having fresh striae superadded in the intervals, 
towards the margin, to supply the vacancies which would necessarily 
be left by the diverging of straight rays, passing from the centre to the 
circumference. A sketch of the manner in which the rays of this 
trochites are disposed is given Plate XIII. Fig. 14, as taken by the aid 
of a lens. 
On viewing these trochitae, when united together in the entrochus, or 
vertebral column, with the aid of a strong magnifier, the articulation will 
be seen to result from the reciprocal reception of the eminences of one of 
these bodies into the depressions of the other. This observation is 
applicable to the articulations of entrochi in general. 
The second species of this kind. Fig. l6, has its exceedingly fine rays 
passing directly from the centre to the circumference, and disposed, 
on the one side, on a concave, and on the other, on a corre- 
sponding convex surface. It is figured by Rosinus, Tab. VII. Cap. 
VIII. B. 2.f 
Possessing but one specimen of this trochites, I am unable to as- 
certain the figure of the entrochus, into the formation of which it en- 
ters. Neither is its entrochus noticed by Rosinus : but as the sides 
* Tentaminis de Lithozois ac Lithophylis Prodromiis, P. 64. 
t Tentaminis de Lithozois, P. 70. 
