430 DESCEiPTioK OF \^Polygastnca. 



Genus Odoxtodiscus, (Elir.) — Loriea simple, of two equal orbicular 

 valves ; lenticular in figure, not concatenated, without apertures and 

 septa ; valves furnished with dotted rays, the number of which is 

 determinate, not cellular ; but remarkable in having erect denticles 

 (spines) on their surface. 



Odoniodisd differ from Actinomjcli, to which they are most akin, 

 by their valves being armed with teeth, of which the latter are 

 destitute. 



0. eecentricm. — Granules of disc disposed in eccentric curved rows, 

 indistinctly radiating ; granules about twenty in 1-1 152nd; mostly 

 with twenty-eight closely-set teeth on the margin. Diameter 

 1 -864th. Fossil in Peruvian guano. 



0. Spica. — With forty-eight rays and denticles near the margin of 

 the disc, and nineteen granules in 1-1 152nd. FossO. in Yirginia. 



O. TTranus. — The disc with thirty -two marginal rays and denticles, 

 and nineteen granules in 1.1152nd. Fossil in Yirginia. 



Genus Omphalopelta, (Ehr.) — Loriea simple, of two equal valves, 

 orbicular, not in chains (?) ; divided intenially into cells by im- 

 porfect radiating septa, which alternate vvith depressions on the 

 exterior ; centi'c smooth, aperttu-es obsolete ; spines erect, scattered, 

 opposite, and placed on the upper margin of each side. Habit of 

 Actinopti/chns and Heliopelta ; differs from the former by its lateral 

 spines ; from the latter, by the spines on its upper margin being 

 scattered (varus.) The number of rays, the figure, and the cells, 

 often a=;similate in these three genera ; but the character of the 

 spines constantly differs in them severally. These spines, moreover, 

 by persisting after spontaneous fission, indicate the singular binary 

 conjunction of individuals in the young state. 



All the species approach, in form, Ad inojjfi/ chits scnaniis, which is 

 common in the North Sea, where they arc never present. 



0. areolafa. — With six septa and rays ; radiating arece loosely, 

 obscurely, but uniformly cellular, and but Uttle or not at all depressed ; 

 rays distinct, margin radiate, wide, a single spine on the upper 

 margin of each area of the disc. Diameter 1 -300th. Fossil, 

 Bermuda. 



0. cclhilosa. — With six septa and rays ; the alternating cellular arcic 



