190 MAGNOTIA. 



Genus— MAGNOTIA, Michelin. 1853. 



This genus was established by M. Michelin* for small urchins, closely resembling 

 Polycyphm, but which are distinguished from that genus by the arrangement of the pores 

 in the zones. The genus is thus defined by its learned author : 



Test elevated, inflated, with a concave base, and numerous small, equal-sized, imper- 

 forate and uncrenulated tubercles in both areas. 



The pores are disposed in single pairs from the disc to the circumference ; from thence 

 to the peristome they become crowded, and form many series. 



The mouth is very large, and occupies much of the base ; the peristome is unequally 

 decagonal, and, from the width of the ambulacral lobes, is almost pentagonal. 



The apical disc is small, and the vent round. 



The ambulacral areas are narrow; the inter-ambulacral areas have a deep median 

 depression, so that the test is divided into fifteen unequal lobes, of which the five ambu- 

 lacral are the smallest. In both areas the tubercles are arranged in oblique ranks. 



The distinctive structural character between Magnotia and Polycyphm consists in the 

 arrangement of the pores, which are in simple pairs in Magnotia, and form triple oblique 

 pairs in Polycyphm : in all other respects these urchins closely resemble each other. 



The only English species of this small group was referred by me to the genus Arbacia 

 of Agassiz, when I first described it in my ' Memoirs on the Cidaridae of the Oolites ;'f but 

 as there is much doubt about the true characters of that genus, I have placed it in 

 M. Michelin's Magnotia, of which it forms a good type. 



Arbacia was a small group of the Echinid^e, proposed by Dr. J. E. Gray, in October, 

 1835,| for a section of the genus Echinus, now living, in which, according to this author, 

 the body is depressed, the ambulacral areas are very narrow, the poriferous zones narrow 

 and straight, the pores in simple pairs, the ovarian and inter-ovarian plates middle sized, 

 and the anus covered by four valves. The types of the genus cited by the author are 

 Echinus pustulosus, Leske, and Echinus punctalatus, Lamk. 



M. Desmoulins,§ in 1834, had proposed the genus Echinocidaris for the same group, 

 and which he thus defined : " General form perfectly regular, circular ; upper surface 

 depressed ; under surface slightly concave ; areas very unequal ; ambulacra always less 

 than one half the width of the inter-ambulacra ; ambulacra complete, lanceolate, straight, 



* 'Revue et Magasin de Zoologie,' No. 1, 1853. 



-j- 'Annals and Magazine of Natural History,' 2d series, vol. viii, p. 278. 



\ ' Proceedings of the Zoological Society,' part iii, p. 58 ; and ' Philosophical Magazine,' 3d series, 

 vol vii, p. 329, Oct., 1835. 



§ 'Tableau analytique des genres d'Eehinides,' July, 1834. ' Etudes sur les Echinides,' p. 14. 



