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larly disposed in concentric circles, and, at the same time, in 

 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 employs 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 onits articular surface. 



The eighth species of trochites, Fig. 19, 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. n. y 



