180 On Cyrloma, a new genus of Fossil Echinida. 



duals, without any corresponding variation in the altitude, 

 as supposed by Dr. Grant and M. Agassiz. Fig. 3, c. c. 

 c. c. c. is an upper view of the disc, shewing the fissures by 

 which the testa is divided. Figs. 4, 5, represent the 

 inner surface of one of the separated divisions ; fig. 6, 

 showing the structure of the testa at the base, and fig. 7, 

 at the apex, when ground to a transparent film ; d. d. the 

 layers of deposit or growth of the anambulacral plates ; 

 8, a vertical, and 9, a transverse section also reduced in 

 the same manner to transparent films, for the purpose of 

 showing the layers of deposit ; fig. 10 the mouth. 



I. — Echinus concretus,* J. M. t. iii. f. 1, 2, 3. 



Shell spheroidal, below rather flat, about 18 anambulacral 

 plates and spines in each vertical series, three spines to each 

 plate placed in oblique rows, the largest spine situated in the cen- 

 tre of each series. Testa thick, strong, and solid. 



Size from 2 to 4| inches in diameter. 



Hab. Bay of Bengal. 



II. — Echinus sulcatus, J. M. t. iv. f. 1, 2. 



Shell spheroidal, below depressed, and recurved round the 

 mouth, apex elevated, about 22 anambulacral plates and spines in 

 each vertical series. Three spines on each plate, above the disc 

 the plates are divided horizontally from each other by a furrow, 

 below they are consolidated. Ambulacra narrow. Alveolus, (fig. 

 2 a,) thin, continuous, and divergent. 



Ordinary size about 1.3 inch in diameter. 



Hab. Bay of Bengal. 



III. — Echinus alveatus, J. M. t. iv. f. 3, 4. 



Shell spheroidal, below depressed, and recurved round the 



* Authors in general think it sufficient to characterise the different genera of 

 Echinodennata, contenting themselves with merely enumerating lists of species 

 from rare works of reference in which they have been figured ; such works indeed 

 as are only to be had recourse to in the great national libraries of Europe. La- 

 marck's " Histoire Naturelle Des Animaux sans Vertebres" contains numerous 

 descriptions of species, but these are for the most part insufficient ; the form, the 

 colour, and the spines which are lost from the bleached shell, constitute the principal 

 characters enumerated in that work, which is also without figures. Under these 

 circumstances I have described the above species from characters presented by 

 the bleached testa, the part commonly met with, without regard to their biblio- 

 graphy. Should any of them prove to have been figured and described before, 

 the names I have given to them may be rejected. 



