78 NAVICELLA. 



also the projection of the apex varies so that it may reasonably be 

 considered as effected by environment; again, the central projection of 

 the septum, usually conspicuous in N. Freycineti, is sometimes want- 

 ing in that species, whilst it occasionally occurs in others. Thus the 

 sectional characters (made subgeneric by Messrs. Adams), insensibly 

 coalesce. As to coloring, certain patterns have been regarded as 

 characteristic of species, but there appear to be no breaks in the 

 modifications by which most of the species may be connected. 



In the interest of scientists (and so, perhaps, of science) it becomes 

 necessary to set up some arbitrary landmarks, be they varieties, 

 species or sections, it matters not, for the recognition of certain 

 stages- of the variations else in unbroken progresssion, and to these 

 stages the names given by naturalists at a time when species were 

 entities have been applied. This task Dr. von Martens has per- 

 formed as well as his opportunities, learning and experienced judg- 

 ment permitted : if I have succeeded in making any improvement, 

 it is due to having his work as a foundation. 



N. BORBONICA, Bory. PL 27, figs. 2-12. 



Rather convex, apex small, somewhat attenuated, produced be- 

 yond the posterior margin ; color varying from dark chestnut brown, 

 unicolored to light olivaceous brown, or tinged with purple and 

 marked with radiating reticulations, interior bluish or blackish, 

 the posterior margin usually with a long black stain on either side. 



L. 25-40 mill. 



Mauritius, Ins. Bourbon, Madagascar, Seychelles Is. 



It is N. elliptica, Lam., (fig. 3) N. tabernaculata, Montf., N. por- 

 cellana, Reeluz, not Linn., N. Cookii, Recluz (figs. 4, 5), N. bimac- 

 ulata, Reeve (fig. 6), var. triloba, Martens=J\ 7 . apiata, Sowb. in part 

 (fig. 7), N. livida, Rve. (fig. 9), very similar to var triloba, var. com- 

 pressa, Martens = N. affinis, Reeve (fig. 8). I do not separate as 

 such the two varieties given by Dr. von Martens, because such forms 

 occur in all the species and are the result of station upon the growth of 

 the individual. The black markings which Reeve has commemorated 

 in his name bimaculata are very usually, but not always present, and 

 unfortunately occur in other species. A more depressed form has 

 been treated by von Martens as a distinct species, but seems to con- 

 nect insensibly with Borbonica-, it is called N. depressa, Lesson, 

 (figs. 10, 11), and N. zebra, Lesson, N. haustrum, Reeve (fig. 12) and 



