S. W. Ford—The Genus Obolelia. 133 
If we compare the interior of O. crassa with that of O. chro- 
matica, illustrations of which are herewith introduced (figs. 3 
and 4), we shall find that, while there are some notable differ- 
ences, the plan in each is the same. The principal differences 
Fig. 3.—Plan of the interior of the ventral valve of 0. chromatica as made out 
by Mr. Billings; Fig. 4.—Ditto of the dorsal. Fig. 5.—Plan of the interior of 
the ventral valve of 0. gemma Billings. The notation is the same for all of the 
gures: aa, cardinal, cc, central and dd, lateral impressions; g, the groove in 
the area; 7, pit in which the groove terminates 
are (1) that the cardinals of the ventral valve of O. crassa have 
their apices directed inward instead of outward as in 0. chro- 
matica ; (2) that the central scars are not here connected with 
the laterals; and (8) that the central impressions of the dorsal 
19) 
same, 
n 
referred to, Mr. Billings includes in his genus, O. chromatica, 
