ON THE GENUS POLYMORPHINA. 215 
series may be said to commence with P. gibba, a subglobular shell, with three or four 
visible chambers, the sutures marked only by lines. By degrees these characters are 
modified in successive specimens, till in P. lactea the shell is rather longer proportion- 
ally, the external chambers number four or five, and the sutures are slightly excavated. 
Again, P. communis, with about the same number of chambers, has the sutures more 
deeply set, and the shell consequently somewhat irregular, but showing better evidence 
of its triserial arrangement. Lastly, in P. problema the number of chambers is increased 
to five, six, eight, or more, the convexity of each is heightened, and their order can 
sometimes scarcely be traced. 
We have endeavoured to place the forms pertaining to this series, described by various 
writers, under one or other of these four subtypes, admitting the more or less compressed 
modifications of P. lactea and P. gibba to a sort of subvarietal distinction. Hence, under 
each, a considerable range of variation in minor and unimportant particulars is com- 
prehended. 
Globulina Caribea of D'Orbigny has a slightly irregular shell, the lower portion of 
which is rugose: P. Muensteri, Reuss, is a porous or punctate variety; and P. ovulum, 
in the same memoir, is inequilateral; P. Roemeri, P. deformata, and P. turgida seem 
to represent heavier, less elegant forms, with the segments irregularly combined. 
From the typical Polymorphina lactea, with its nearly circular transverse section, may 
be traced a series of modifications tending towards the complanate forms; and the refer- 
ences to the various authors who have alluded to these are placed together in the second 
division of the above synonymy. The subjoined outlines, taken from figures of four such 
subvarieties, about equidistant from each other in the chain of differentiation, will show 
how slight is the basis on which they have received distinctive names. Specimens inter- 
mediate again to these could, without difficulty, be found. 
Polymorphina lactea, var. amygdaloides, Reuss. 
je 9.9 Q0 6: 
P. minuta. P, guttula. P. depauperata. P. amygdaloides. 
P. sororia of Reuss is one of the most compact of the flattened varieties, elongate and 
almost pyruline in septation. P. minuta (a) scarcely differs from the type, except in its 
lateral compression. P. guttula (b) and P. depauperata (c) present the varietal peculiarity 
in increased degree. Lastly, in P. amygdaloides (d) we have a close approximation in 
contour to P. oblonga (Plate XXXIX. fig. 7) ; and it may even be a question whether this 
latter “ species” might not with propriety have been brought into the same series. 
P. inequalis and P. deplanata are less regular and less symmetrical modifications of 
the type, the latter somewhat resembling P. Burdigalensis. 
- Distribution.—The range of Polymorphina lactea is very wide, whether regarded as a 
question of time or of geographical area. We have specimens from the Kimmeridge 
