4 



BKITISH FOSSILS. 



plates are heptagonal, with a broad indented base, and pierced near 

 their outer margin by the ovarian pore ; the five oculars between 

 them are small pentagons pierced also near their base or outer edge. 

 A circle drawn round the plates of the disk would touch or nearly 

 touch the outer edges of all, excepting the madreporiform plate, 

 which is generally swelled as well as enlarged, at least in the pent- 

 agonal variety. 



Variations. — If all the specimens on our plate be referable to 

 the same species, the species is a very variable one indeed. It 

 varies in height, — from a rotund to a pentagonal and even sub- 

 lobed form, — and very greatly in the degree of development of 

 its tubercles and of the gTanules of the test. These last are, in the 

 typical foreign examples, of considerable prominence, so much so as 

 partly to obscure the primary rows ; while in our British examples, 

 though we have a few that show as many secondary tubercles, yet 

 these last are always smaller, and in the extreme variety (Forbesii, 

 fig. 6, if variety it be), the surface is pretty uniformly covered with 

 small granules, amongst which the rows of large tubercles are con- 

 spicuous, both on the interambulacral and ambulacral areas. This 

 kind of arrangement gives its character also to the next variety, 

 called serialis by Agassiz ; the large tubercles are not in reality 

 more prominent than in other varieties, but they appear so fi:om 

 the want of secondary tubercles among them. 



Var. Forhesii, fig. 6. — Besides the circular inflated form, in which 

 it resembles JE. perlatus, and difiers from the variety above de- 

 scribed, a view of the underside of this variety shows a distinction 

 in the mouth, which Prof. Forbes has noted in his MSS. : " The 

 mouth notches are acute,'' he says, in ^. perlatus (the pentagonal 

 form), obtuse in E. serialis^' (our figured specimen, fig. 6). Moreover, 

 in E. perlatus the pairs of pores are much less oblique, so that the 

 series become parallel on the upper surface of the test as well as 

 below; whereas in serialis the pores are only subparallel, beginning 

 from the fourth row of pores from the mouth.'' This is a character 

 which occurs in the young of other varieties, but only near the 

 apex. Probably older specimens of the \ar. Forhesii would have 

 the pores more parallel. The avenues are very little sunk either on 

 the upper or lower face of the test, and the interambulacral spaces 

 not at all indented along their middle. The arrangement of the 

 tubercles is much the same, as in var. germinans, but in the outer 



* Prof E. ¥orbes regarded this variety as the E. serialis of Agassiz ; but for the 

 reasons assigned in the next page it is here described as var. Forhesii. 



