BRITISH FOSSILS. • 9 



Description of the Plate. 



Fig. 1. Specimen of the form /3 seen from above. 

 Fig. 2. The same from below. 

 Fig. 3. The same, posterior view. 

 Fig. 4. The same, lateral view. 



Fig. 5. Ambulacral and interambulacral plates, showing the termination of the dorsal 

 petals of one of the lateral ambulacra. 



Fig. 6. Spiniferous tubercles of dorsal interambulacral plates* 

 Fig. 7. Ditto of ventral plates. 



Fig. 8. Base of one of the straight spines, greatly magnified. 



Fig. 9. Outline of a young specimen of , the elongated form (in Mr. Bowerbank's 

 collection.) 



Fig. 10. Outline profile of the normal fornii. 



Fig. 11. Outline profile of the gibbous form (in Mr. Bowerbank's collection.) 

 Fig. 12. Ditto of the sub-gibbous form. 



Fig. 13. Granulation and tumidity of the ambulacral plates of the petaloid portion of 

 the dorsal ambulacra (highly magnified). This is the distinctive character of the species. 



Fig. 14. A spine from near the anal region, figured from a specimen in Mr. Bower- 

 bank's collection : the spatulate extremity is shown by its side. 



Fig. 15. A portion of a spine, exhibiting a spirally rugose structure. 



Note on Allied British Species. 



Mr. Morris, in his valuable "Catalogue of British Fossils" (1843), enumerates ten 

 species of Micraster, Since the publication of that work the Brissoid species of the group, 

 as then received, have been separated very properly by Agassiz. Only four of the names 

 there given, consequently, fall strictly under Micraster ; viz., cor-anguinum, cor-iestudi- 

 narium, gihbus (for which both Lamarck and Goldfuss are cited), and rostratus. All of 

 them are here regarded as varieties of one species, viz., Micraster cor-anguinum. 



There is, however, a Micraster found in our British chalk, which is very distinct from 

 cor-anguinum, and which does not appear to belong to any of the described species of 

 foreign authors. I have given an account of it in Mr. Dixon's work on the Geology of 

 Sussex, where it is excellently figured. I have there named it Micraster cor-bovis. It is 

 usually a larger and longer species than cor-anguinum^ and its petaloid ambulacra are 

 more deeply impressed and much shorter in proportion to the body. The shape is ovato- 

 cordate, the curve of the sides from the front of the antero-lateral ambulacra to the anal 

 extremity being but slight, its chief swelling being near the anus, and not on a line with 

 the end of the postero-lateral ambulacra, as in cor-anguinum. The back is more equally 

 depressed than in the depressed variety of the last-named species. The mouth is much 

 smaller comparatively, and the post-oral spinous space, though much longer, in consequence 

 of the elongation of the hinder portion of the test, is nevertheless proportionally broader. 

 The tubercles of the plates, whether dorsal or ventral, are much smaller and more scattered. 

 Besides all these comparative characters, there is the positive one that in cor-bovis the 

 ambulacral plates, instead of being tumid are smooth and plane, as are also the ridges 

 separating the sulcations of the pairs of pores in the petaloid ambulacra. The ambulacral 

 spaces are wider than the breadth of any of the sulcations. As contrasted with Cor- 

 anguinum, the diagnosis of cor-bovis would stand thus : — 



[ill. X.") C 



