374 PROCKKDINGS OF THE NATIONAL MUSEUM. vol.xxi. 



one valve will be pauciradiiite, the other iimltiradiiite; again, in one 

 valve the rays may be narrow and in the other marked with broad rays. 

 Another aspect is seen with a few rays on one valve; on the other nar- 

 row and broad rays alternating, often with wide interspaces of the 

 gronnd (;oloi'. 



Variety f*>r^\ serialis. 



The foregoing variety might well be regarded as including this, 

 thongh sei)arat('d by me so as to include examples wherever the rays 

 and ground color are of the same tints and shades, but the rays are 

 arranged in a somewhat serial order, the two valves being unlike, how- 

 ever, as to tlie ])osition of the rays and the number thereof. 



These segregations rf and 66 will include neiirly all that remain of 

 the examples, inclusive, under the general head of Group I, that do 

 not fall into tlu^ ])revious varietal sections. 



Variety t, interrupta. 



This is an ex(;eedingly rare aspect; only 1 in oOO; the ground color 

 dull white at Mie umbos, brownish or dark cream below, a i'ew rays of 

 pale rufous brown and these obsolete in the umboual region and inter- 

 rupted below. 



Variety ', luteobrunnea. 



Ground color yellowish brown in the umbonal region, warm dark, 

 rufous, with purplish zones and tint below, and narrow and broad rays 

 of pale i)uri)]is]i brown. A very lare variety; only 1 in 500. 



GROUP II. 



lAfflit nn/,s <m <t light (fronnd. 



Variety ;/, uniradiata. 



in this variety, which is the commonest of the group, the ground color 

 is usually tlu^ same as that of the ty]>e, though often somewhat darker, a 

 pule umber or i)ur[»lisli asli, for instance, whichever maybe the general 

 tone of the surface; an anterior single paler ray extends from the umbo, 

 gradually widening as it approaches the ventral margin; this ray 

 closely adjoins and follows next to the anterior angle of the valves; 

 otherwise this variety is usually plain, though occasionally marked with 

 zones of pale purple or pale umber or a combination of these two tints. 



Variety 0, biradiata. 



This is a comparatively rare variety, with the ground tints the same 

 as in the preceding. In addition to the anterior ray extending to the 

 ventral edge of the valves, there is a short ray in the opposite direc- 

 tion. The umbonal region in this variety is usually pale ashen blue or 

 faint dull reddish purple. 



