^N. tessellata^ is a smaller species, with heavy revolving ribs, and is further decorated with transverse oblique black lines. The operculum is calcareous. Both of these species have the habit of absorbing the entire inner portion of their shells. They belong to the West Indian fauna, and occur in great numbers on rocky or coral shore stations. They also may be found on the east Florida coast, well to the south. A third species, ^N. versicolor^, often occurs, associated with the other two. It is somewhat smaller than the others and can easily be distinguished by the four teeth on the columellar lip, the edge of the columella being convex. (Plate LXVII.) GENUS ^Neritina^ The genus /Neritina/ is very closely allied to the last, having quite the same form of shell; but it is usually more globular and [pg364] variously ornamented by bright spots or zigzag lines of coloration. 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 volcanic 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 ~JANTHINIDÆ~ GENUS ^Janthina^