30 Study <tf N. obsoleta anfJ N. tn'viftata 



has noted that where closely related forms live in brackish water and in sea water, 

 the forms in brackish water will in general have a less solid shell tlian the forms 

 in sea water, yet as the wator becomes still more dilute, the forms inhabitinj; it 

 have more solid sliells. Possibly there are conditions of the sanie kind holding 

 for the size and shape of the shells from the three localities. The stunting 

 influence, for example, may be checked to some degree in the inner harbor, 

 because there the food supply is most abundant. 



The effect of density of water on the size and shape of animals living therein 

 has been often noticed. Bronn ('G2), in a section on the effect of salt in the 

 water, states that in the Baltic, contaiiiing only 4 — 6 p.m. .salt, Buccinuin iindutuin 

 and Littorina litorea live, but are always small and stunted. In the Mediterranean, 

 containing .37 p.m. salt, oysters are small; at 30 — 20 p.m. they are, if not largest, 

 of the best fiavor ; and at 18 — 17 p.m. they are stunted: which would seem 

 to show an optimum content of salt, above and below which the animals are 

 dwarfed. Simroth ('91) gives as the peculiarity of brackish watcr fanna that 

 fewer species but more individuals are present ; and quotes Möbius and Heineke 

 to the effect that fewer species of smallei- size are characteristic. Walther ('93) 

 says that the less dense water of the Baltic dwarfs many of the animals living 

 therein, and he gives illustrations among mollusks and worms. Bumpus ('98) 

 in an article on the variations of Littorina litorea gives statistics that show that 

 the more dense the water the less globose are the shells of the animals living 

 therein. The means of the curves given by Bumpus in hi.s paper, as calculated by 

 Duncker ('98), express more nearly the condition of the wliole population than do 

 Bumpus's own figvn-es, and are therefore used in comparison. The water at 

 Bristol Narrows and Wan-en River, in Rhode Island, is less dense than the water 

 at Seaconnet and Newport. The globosities are 90'77 7, and 91*08 7o ^^ Bristol 

 Narrows, 92-69 7, at Warren River, 89-72 7, at Seaconnet, and 89-18 7, at 

 Newport ; showing decidedly greater globo.sity in the less dense water. The data 

 as to relative densities of the water in other localities from which Bumpus coUected 

 shells are not given, so no further conclusions can be drawn. The foregoing 

 evidence is in accord with the observations noted above on N. obsoleta. 



If the means of the different eharacteristics of N. trivittata be compared with 

 the means of N. obsoleta, the observations that were made qualitatively will be 

 verified. The shells of N. trivittata are rougher, smaller, less globose than the 

 shells of N. obsoleta, and with a more acute angle at the apex. The ränge of 

 each character mesisured overlaps considerably, and evcn tho means are not 

 very far apart. 



(7) Comparison of Cold Spring Harbor type with tite normal type. If we 

 compare the dimensions of the Nassa of Cold Spring Harbor with t^-pical 

 measurements given by Apgar ('91) and Gould ('70) for the two species, we find 

 various interesting differenccs. After looking, by the courtesy of Professor 

 Pilsbury, over the collection of Nassa shells from different localities in the 

 museum of the Philadelphia Academy of Sciences, it seemed fair to accept an 



