78 NILS HJ. ODHNER, STUDIES ON RECENT CHAMIDAE. 



In the same respects it differs from the other West Indian form that can be thought 

 of, Ch. macerophylla. For these reasons it seems probable that Ch. lamarckiana is 

 a large form of Ch. sinuosa; in order to be quite certain about this it is, however, 

 necessary to examine small specimens of that form. 



To growth are also to be ascribed changes in the dentition observable in some 

 cases. By the successive deposition of lime substance effectuated by the animal it- 

 self the hinge elements grow by degree coarse and indistinct; above all tooth 3 a 

 shows a tendency to be obliterated as 3 b increases. 



We now have considered some cases of variability caused by oecological facts. 

 This is, however, certainly complicated by the interference of a variation which has 

 its grounds in genetical conditions and is subject to mendelism. I only wish to 

 draw attention to the matter without entering on a discussion of it. A variation 

 of this kind is manifested, for instance, in the shape and the position (distance) of 

 the addnctors and their relation to the pallial line, the nature of the margins of 

 the shell (if smooth or crenulated) and of the inside of the shell (if smooth, grained 

 or striped), the colour and other characteristics, Avhich may be differently developed 

 in different specimens of the same species. It should be observed, however, that 

 in some Chamas the same characteristics are specifically fixed and constant as vary 

 greatly in other species of the genus. Thus, for example, the relative size of the 

 adductors in Gh. coralloides is fixed inasmuch as the anterior one is twice as long 

 as the posterior one, while some variation in this relation is observable in other 

 species. In Gh. macerophylla and crenulata the adductor scars are situated entireiy 

 inside the pallial line, which passes outside them; in other species, e. g. Ch. sarda, 

 the pallial line coincides more or less with the outer börder of the adductor scars, 

 which are generally situated inside this line; in Ch. iostoma (= rilppelli M' Andrew, 

 not Reeve) they are placed half-way out of the pallial line. Another example is 

 the fact that in Ch. ynacerophylla the börder of the shell outside the pallial line is 

 narrower than half the adductor breadth, while in Ch. sinuata the same zone is 

 broader than half the adductor; in other species this characteristic varies. Lastly 

 we find that in Ch. macerophylla the scars of the branchial snspensor muscles are 

 very distinct in every specimen; in Ch. gryphoides, for example, they are, on the 

 contrary, sometimes not visible at all. 



In spite of the great variability of the characters of the shell it still seems 

 possible to state some main points with regard to a classification of the Chamas on 

 the basis of their sculpture. This, too, varies greatly, it is true, due to surroundings, 

 growth, wear and other external conditions, but nevertheless the sculpture presents 

 characters of specific constancy. Ch. sinuosa, for example, is at once recognized by 

 its generally deeply impressed furrow running from the umbones to the back margin, 

 and this furrow is always more or less distinct, however much the sculpture may 

 vary in other respects; it appears even in worn specimens, which are identifiable 

 because of this. On the other hand Ch. brassica and its allies are easily identified 

 by the scaleless zone near the postero-dorsal margin. If characteristics may thus 

 be given which are significant for single species, it is also possible, to some extent 



