25 



suffered the least injury. It formed a part of Mr. Forster's collection; and 

 is, I conjecture, from Oxfordshire. It needs no other notice of its form, 

 than that it is nearly circular, deviating only in having that part of the 

 shell, which is the region of the anus, a little more produced. It not 

 having been thought necessary to figure the inferior surface, it is proper 

 to observe, that it is slightly concave, the mouth being pentagonal, and 

 the vent oval and transverse, and situated near the margin ; but more on 

 the under side, than it appears in the figure. 



The echini of the fourth section, CATOCYSTI, are named Placenta, by 

 Klein, the shells being flat, like a cake, and variously formed. They are 

 all ornamented with a pentaphylloidal flower. The mouth is in the mid- 

 dle of the base, and the anus near to the margin, or to the third region of 

 the axis. This section is divided by Klein into thfee genera, mellita, 

 laganum, and rotula, which are comprised by Leske in the genus Echi- 

 nodiscus, so named by Breynius. The characters of this genus are, a 

 depressed, discoidal figure, nearly flat on both sides; ambulacra imi- 

 tating the forms of petals ; a smooth central mouth with teeth ; the top 

 perforated with four large pores. 



The Melita, honey-cake of Klein, forms the first family of this genus. 

 The species differ in the number of foramina, the situation and the form 

 of the periphery. 



Echinodiscus bisperforatus, E. quinquiesperforatus, E. sexiesperforatus, 

 E. emarginatus, E. auritus, E. inauritus, E. quaterperforatus, do not appear 

 to have been hitherto known in a petrified state. The fossil which is 

 represented Plate II. Fig. 6, is the only fossil which I have seen of this 

 family, and is undoubtedly one of the first species, Echinodiscus bisperfo- 

 ratus, as will appear, on a comparison with Klein's figure of the recent 

 shell, Tab. xxi. A. B. and with Leske's description of that part, which is 

 here in best preservation. " Ambulacra petalorum ovatorum figuram 

 sistunt, atque singula ex serie simplici punctorum interiore, et striarum 

 obliquarum, testam penetrantium, non tamen puncta interiora prorsus 

 attingentiam, componuntur : in apice ambulacrorum puncta non con- 



VOL. III. E 



