364 MARINE INVERTEBRATES 
variously ornamented by bright spots or zigzag lines of colora- 
tion. The neritinas have acquired the habit of ascending rivers, 
until they have become almost wholly a brackish- or fresh-water 
genus. The metropolis of this genus is in the South Sea Islands, 
where it attains a wonderful development in the clear running 
streams of the voleanic islands. 
N. reclivata. A very pretty olive-green species, with very fine, wavy, 
longitudinal lines of coloration, found in almost all Floridian streams 
above the action of the tide. The nacre of the columella and within 
the aperture is bluish-green, and the operculum is rich olive. (Plate 
LXVI.) 
N. viridis. A small, intensely green species, which, unlike most 
neritinas, is strictly marine. It belongs to the great West Indian faunal 
province, but is occasionally found upon the shores of Florida and of 
Texas. 
The prosobranch gasteropods thus far considered (belonging to 
the suborder Diatocardia) all show by their anatomical structure 
that the process of visceral torsion, though carried very far within 
them, has not been complete enough to crowd out and finally 
destroy one of each of the paired organs. Both auricles of the 
heart (with some exceptions) were left intact. 
SUBORDER MONOTOCARDIA 
In this the second suborder of the prosobranch gasteropods, 
the twisting visceral process has been carried to the extreme. 
The heart has but one auricle. There is but one gill (on the left 
side), and this is attached to the inner side of the mantle flap. 
In other respects the presence of certain specialized organs would 
probably indicate that gasteropods belonging to this suborder 
are a step higher in the scale of life than those which belong to 
the Diatocardia. 
FAMILY JANTHINIDE 
Genus Janthina 
The genus Janthina has an exceedingly thin and semi-trans- 
parent shell, deep violet in color on the base and lighter blue on 
the spire. An interesting feature of this genus is the mode of de- 
positing the ova. The female exudes from a gland in the foot a 
