161 
larly disposed in concentric circles, and, at the same time, in rays, 
converging from the circumference to the centre. 
The numerous different markings of figures, observable on the flat 
surfaces of the trochitae, shew what various means nature some- 
times emplo 3 ^s to produce a similar effect : thus increasing the beauty 
of her productions, by an unceasing variety of forms. In no instance, 
hardly, is this sportive luxuriance more evident, than in the en- 
trochite. Fig. 18. It is there seen, that the striae are more thickly 
disposed in some places than in others. Some of these striae pass di- 
rectly from the centre to the circumference ; others pass through only 
half that space. Some are undivided, whilst others are bifurcated ; 
the whole forming a surface singularly beautiful in its appearance, 
and well fitted for the office for which it was formed. This trochites is 
represented nearly double its size, that the disposition of its striae 
may be better seen. 
The sixth species. Fig. 33, is rather uncommon ; it is beautifully 
marked over the whole of each surface, with undulating and ramify- 
ing ridges and correspondent depressions, elegantly disposed from 
the centre to the margin, where they terminate in exactly regular dis- 
tances. This trochites is represented of twice its natural size, the 
better to shew the curious forms on its articular surface. 
The eighth species of trochites, Fig. IQ, I believe also to be rare. 
It is perfectly circular ; and from its circular central foramen pro- 
ceed five phylloidal rays, which terminate in equal distances, in a 
narrow crenated margin. 
The trochitae with a stellated central foramen, as well as those with 
a circular foramen, may be divided into several species. The first of 
these. Fig. 20, is closely beset with very fine striae, passing directly from 
the centre to the circumference, in a similar manner with those of the 
trochites. Fig. 13. The central foramen of all the following specimens 
are pentagonal, except where otherwise described. In the second spe- 
VOL. II. Y 
