from the Lias and Oolites. 313 



nences in the centre of the plates; the margins of the 

 areolas surrounded with circles of small granules ; no second- 

 ary tubercles. 



Height /oths of an inch, transverse diameter ^^ths of an inch. 



Description. — This Pedina presents a different form from its 

 other congeners : the test is circular and depressed ; the ambu- 

 lacral areas are narrow, about one-third the width of the inter- 

 ambulacra; the usual double row of tubercles in this region is 

 reduced to one row, the tubercles of which are disposed alter- 

 nately on the right and left sides of the areas, thereby forming 

 a single zigzag line down the centre thereof; the tubercles at 

 the equator and on the upper surface are small, but there are 

 two or three of a larger size at the base of the areas ; a few gra- 

 nules form imperfect crescents round their narrow areolas. The 

 interambulacral areas are nearly three times the width of the 

 ambulacra ; they are adorned with five pairs of primary tuber- 

 cles of nearly a uniform size throughout, which are raised on 

 prominent mammillary eminences, the summits of which ai*e 

 smooth, ring-like and without crenulations ; circles of small gra- 

 nules bound the areolar spaces ; there are no secondary tubercles, 

 nor any sculpture upon the intertubercular surface of the plates, 

 so that down the centre of the areas there is a smooth valley 

 between the primary tubercles. The apical disc is well preserved 

 in our specimen ; the ovarial plates are of an irregular octagonal 

 form and of moderate size, they are covered with a few granules 

 scattered irregularly over their surface ; the ocular plates are of 

 a rhomboidal form and have large eye-holes. The base of the 

 specimen is covered up with hard rock, so that it is impossible 

 to expose the mouth-opening without endangering the specimen. 



Affinities and differences. — The Pedime have been so imper- 

 fectly described by M. Agassiz in his ' Descriptions des Echi- 

 nodermes fossiles de la Suisse,' that there is much difficulty in 

 making out the species figured and described in that monograph. 

 When it is recollected how limited were the materials at Agassiz's 

 command when he published that valuable contribution to 

 Palseontology, and how delicate the test of this genus is, we can 

 readily understand how so many diflFerent forms of the same 

 Urchin came to be described and named as distinct species. 

 After a careful examination of many specimens, we confess that 

 Pedina aspera, rotata, ornata and subUevis, Agass., appear to us 

 to be so many dififerent forms of one and the same species. We 

 have before us likewise the original type specimen of P. granu- 

 losa, Ag., which has been kindly communicated by Professor Des- 

 longchamps. An examination of that Urchin has convinced us 

 that it is only a larger individual of P. aspera, as we find it in 



