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 volcanic islands. 



N. reclivata. A very pretty olive-green species, with very fine, wavy, 

 longitudinal linos of coloration, found in almost all Ploridian 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 MONOTOCABDIA 



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 JANTHDTCDJE 



GENUS Janthina 



The genus Jantliina 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. 



