159 



broader surfaces. Trochitae, with a circular hole, may be divided into 

 the following species : 



The first species has strise passing nearly straight, from the centre 

 to the circumference. 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 striae 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 arranged exactly as might be expected, the same stride 

 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 articula- 

 tion will be seen to result from the reciprocal reception of the emi- 

 nences 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. 15, has its exceedingly fine 

 rays passing directly from the centre to the circumference, arid dis- 

 posed, on the one side, on a concave, and on the other, on a corres- 

 ponding convex surface. It is figured by Rosinus, Tab. VII. Cap, 

 VIII. B. 2 t- 



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 Prodromus, p. 64. 

 f Tentaminis de Lithozois, p. 70. 



