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192 differences among individuals whose conditions of existence are very diverse. Hence the analogy of Operculina affords good grounds to surmise that many of the reputed species in the nearly-allied genus Nummulites have no real title to that rant; the differences among many of them being not nearly so great as those we have met with among the varieties of Operculina; whilst those presented by many others do not exceed what might be reasonably expected to occur under a greater variety of modifying agencies. But I have shown that it may be fairly questioned whether there is adequate ground for upholding the generic distinctness of Operculina and Nummiilites; the characteristic by which the latter has been asserted to be specially distinguished, being not unfrequently observable as a varietal difference in the former. The form which I have described under the designation of Amphistegina Cumingii, bears a striking resemblance to the ordinary Nummuline type in the early part of its growth, and to the ordinary Operculine in the later; and may be regarded as in many respects a connecting link between the two.

There appears, then, strong reason for considering Cycloclypeus, Heterostegina, Operculina, Nummulites, and Amphistegina as related to each other in the same manner and degree as the leading forms already enumerated under the Orbiculine group. And it is very curious to observe the perfect analogy which prevails in regard to the forms under which these two great types of structure—essentially different as they are—tend to develope themselves. As I have already pointed out, the relation of Cycloclypeus to Heterostegina is exactly that of Orbitolites to Orbiculina. So, if the transverse or secondary septa of Heterostegina were undeveloped, we should have an Operculina, Nummulina, or Amphistegina (these three types being, in my view, essentially one and the same), just as the like deficiency actually occurring in Orbiculina gives to it all the essential characters of Peneroplis. And the parallelism seems to be completed by the existence in Fusulina of the same metamorphic condition of this type, that Alveolina is of the Orbiculine. The accordance of all these in the highly elaborated texture of the shell, in the relation which this bears to the segments of the sarcode-body, and in the presence of an intermediate skeleton with its canal system, is extremely close. The substance of the shell is very dense, and of almost vitreous transparence where it is not perforated by the minute closely-set tubuli, which usually pass direct from the interior of the chambers towards the external surface. Each segment of the body has its own proper envelope, so that the septa between the chambers are composed