Notes on Carboniferous Lamellibranchs. 357 
In many respects these small Lamellibranchs show points 
of resemblance to Plewrophorus, and I am diposed to group 
them along with that genus under the Carditide. 
Considerable diversity in external form may be noted in 
the examination of a large series of individuals. Taking 
external form as a basis for classification into species, we 
find the individuals in a large series of specimens ranging 
in two divisions. At one end of the series are those 
characterised by a sub-quadrate or a sub-rhombic contour 
of the shell margin; while at the other end of the series 
the same contours are sub-elliptical, or rather sub-ovate. 
Viewing types selected from either extreme of the series, the 
paleontologist might, apparently with good reason, regard 
these as perfectly good species, and as such, accordingly, 
they have generally been regarded, and have been designated 
C. rhombea and C. parallela respectively. But the occurrence 
along with these forms, in the same stratum, of shells 
presenting every intermediate gradation of form between 
the two extremes, is sufficient evidence to prove that we are 
dealing with a single species composed of individuals with a 
wide range of form. 
For this species Phillips’s original specific name of parallela 
may very well be retained; while for the sub-quadrate 
forms occurring at one extreme, the sub-specific or varietal 
name rhombea may be used as a trinomial, if so desired; 
while for the sub-ovate forms constituting the other extreme, 
those who care to mark the variety could employ the 
trinomial ovata, which would then rank as Cypricardella 
parallela var. ovata. 
Part IV. BATHYMETRICAL DISTRIBUTION. 
In dealing with extinct species of marine Invertebrata, 
the geologist ha¥ frequently occasion to search for evidence 
in regard to the conditions of depth under which certain 
species or assemblages of species may be supposed to have 
lived. The direct evidence bearing upon this question is 
not by any means generally satisfactory. But a certain 
amount of indirect evidence is obtainable, and this, in default 
of anything better, may serve, at least temporarily, as some 
