FROM THE INFERIOR OOLITE. 171 



Gems—'PEDmA, Agassiz, 1840. 



The urchins grouped in this genus are sometimes large, but in general they are of 

 moderate size ; their test is much inflated at the sides, and nearly equally depressed on 

 the upper and under surfaces. The shell is extremely thin, and the plates have numerous 

 small tubercles developed on their surface. 



The ambulacral areas are narrow, from one third to one fourth the width of the inter- 

 ambulacral ; they have two rows of marginal tubercles, often as large as those of the other 

 areas. 



The inter-ambulacral areas are wide, with two rows of primary, and two or four rows 

 of secondary tubercles, which extend only from the peristome to the circumference. The 

 tubercles are perforated, the bosses have smooth uncrenulated summits, and the areolas 

 are narrow and superficial. 



The poriferous zones are wider than in the other Diademad^, and the pores are 

 arranged in obhque ranks, with three pairs in each file. In this respect the Pedinas 

 resemble the EcHiNiDiE, and form a connecting link between these two natural families. 



The mouth opening is small, in general from one third to one fourth the diameter of 

 the test; the peristome is divided by narrow superficial notches into ten nearly equal- 

 sized lobes. 



The apical disc is small ; the genital plates are nearly equal, but the right antero- 

 lateral, with the madreporiform body, is a little larger than the others ; the ocular plates 

 are small, and both genitals and oculars are perforated near their outer third. 



The spines, unfortunately, are unknown. 



The Pedinas were first described by Professor Agassiz as having perforated tubercles, 

 with crenulated bosses. " Enfin un dernier caractere de ce genre consiste dans la petitesse 

 de ses tubercles, lesquels cependant sont perfores et mamelonnes comme ceux des 

 Diademes."* 



In their ' Catalogue raisonne des Echinides,'t MM. Agassiz and Desor defined this 

 genus — " Oursins comprimes, a test mince, a bouche petite, pen entaillee. Trois paires 

 de pores obhques. Tubercles perfores et crenules comme chez les Diademes. Toutes les 

 especes sont fossiles ; des terrains Oolitiques et Cretaces." 



The same definition was adopted by M. Cotteau;j and as the specimen I first de- 

 scribed was not well preserved, I repeated § the statement on the authority of these 

 writers. 



The discovery, however, of a number of good specimens of Pedina rotata enabled me 



* ' Echinodermes Fossiles de la Suisse,' 2de partie, p. 33. 



-j- 'Annales des Sciences Naturelles,' 3me serie, tome vi (1846), p. 3/0. 



X 'Etudes des Echinides Fossiles,' p. 191. 



§ ' Annals and Magazine of Natural History,' 2d series, vol. viii, p. 272. 



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