58 



used for this purpose, such as the number of the tesseree, and of the 

 pores in the ambulacra, have been found to be inconstant ; the num- 

 ber of these increasing, as they are now known to do, with the age 

 of the specimens. He proposed to divide the Echini as follows : 



Genus 1, Arbacia. 



Corpus depressum. 



Area ambulacrorum angustissimae : ambulacra angusta, recta, sin- 



gulo e serie simplici tesserarum biporosarum superpositarum 



eiFormato. 

 Tessera ovariales et interovariales mediocres. 

 Anus valvis quatuor spiniferis tectus. 



This genus corresponds with Echinus section A. of M. de Blain- 

 ville, and contains Arbacia pustulosa {Echinus pustulosus, Lam.), 

 Arb. punctulata {Ech, punctulatus. Lam.), &c. 



Genus 2. Salenia. 



Corpus subsphsericum. 



Area ambulacrorum angustissimae: ambulacra angusta, recta, sin- 



gulo e serie simplici tesserarum biporosarum superpositarum 



efformato. 

 Tessera ovariales et interovariales maximse. 

 Anus subexcentricus. 



This genus is known only in the fossil state, and has hitherto even 

 been confounded with Cidaris, but its tubercles are not pierced. It 

 comprehends Salenia scutiger (Cidaris scutiger, Munst., Goldf, Pe- 

 tref., t. 49. f. 4.— Park., Org. Rem., t. 12. f. 13.; Echinus petali- 

 ferus, Desm.) and two or three other allied species in Mr. Gray's 

 collection. 



Genus 3. Echinus. 



Corpus plus minusve depressum. 



Area ambulacrorum latitudine dimidium arearum extraambula- 



cralium aequantes : tessera ambulacrales tripliciter biporosae. 

 Tessera ovariales et interovariales mediocres. 

 Anus subcentralis, squamosus ; squamis spiniferis. 



The ambulacral tessera in this genus may be regarded as being 

 each composed of three doubly pierced pieces : of these the upper 

 is placed in the ipiddle of the upper edge of the tessera ; the next 

 below it, on the njiddle of the outer edge ; and the lowest on the 

 lower part of the inner edge of the plate : so that when the plates 

 are together, forming the ambulacra, the pores appear to form oblique 

 lines, each composed of three double pores, the inner upper double 

 pore of each line belonging to the plate above the other two double 

 pores. 



This genus contains the sections B*. C. E. and G. of M. de Blain- 

 \'ille. The species may be divided into two very distinct sections, 

 thus : 



