2 



BRITISH FOSSILS. 



Var. h, tumidior. Sjpatangus gihhus, Goldfuss, Pet. Germ., p. 156, 

 pi. 48, fig. 4. Spatangus cor-anguinum. Woodward, Geol. Norf., pi. 5, 

 fig. 8. Micr aster gihhus, Agassiz, Cat. Syst., p. 2 ; E. Sismonda, Mem. 

 Ech. Foss. Nizza, p. 23. Micraster hrevis^ Desor, Cat. Rais. des Echin. 

 Ann. des Sc. Nat., 3rd ser., tom. viii., p. 24. 



The first illustration of the genus Micraster selected for these 

 Decades is one of our commonest cretaceous Echinidse. It is chosen 

 partly because the perfection and abundance of specimens permit of its 

 structure being described and delineated more in detail than that of any 

 of its congeners, and partly since, common as it is, nevertheless, like 

 most common fossils, it requires a much more critical examination than 

 it has yet undergone. 



It has been figured and described, according to their fashion, by many 

 of the older writers on fossils. Llhwyd, Hook, Breyn, Schlotheim, 

 Walch, Morton, Klein, Davila, Van Phelsum, and Leske have severally 

 represented and commented upon it. Very variable in its features, un- 

 less we have many specimens before us, we can scarcely put two or three 

 examples together which are closely alike in proportions. This capacity 

 for variation has caused many spurious species to be constructed out 

 of one. 



It is the type of the genus Micraster of Agassiz. That genus is dis- 

 tinguished among the other Spatangacese with petaloid ambulacra, from 

 Spatangus and its allies, by its surface being covered with secondary 

 tubercles only, and by its rather narrow ambulacra, not bounded by or 

 including a fasciole of tertiary spines ; by the presence of a sub-anal 

 fasciole from Brissus and Amphidetus. 



The outline of the body is always cordate, in most specimens nearly 

 as broad on the widest part as long, sometimes broader than long — 

 rarely much longer than broad. The greatest width of the lateral out- 

 line is invariably a little behind the antero-lateral ambulacral plates ; 

 from that point to the anal extremity it describes a regular and consi- 

 derable curve in all the varieties. The dorsal surface varies from an 

 almost conical convexity (extreme form of Spatangus gihbus^ Lamarck) 

 to depressed, as seen in the larger variety of the typical cor-anguinum. 

 When very conical and compressed, then the highest part, or vertex, is 

 at the apex, or ovarian, disk of the shell ; in the depressed form it is a 

 little behind this disk ; in the rostrated form it is behind this disk, nearly 

 in the centre of the posterior interambulacral area. The anterior half 

 of the back varies in degree of declivity in all the forms, but is especi- 

 ally steep in the extreme form of gihhus^ and in a not uncommon varia- 

 tion of rostratus. Intermediate specimens, exhibiting every degree of 

 tumidity of the cheeks (or swelling on each side of the odd ambulacrum) 

 are common. As a general rule, the cheeks are tumid. The posterior half 



