192 ORIGINAL ARTICLES. 



differences among individuals whose conditions of existence are very 

 diverse. Hence the analogy of Operculina affords good grounds to sur- 

 mise that many of the reputed species in the nearly-allied genus Nummu- 

 lites 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, He- 

 terostegina, Operculina, Nummulites, and Amphistegina as related to each 

 other in the same manner and degree as the leading forms already enu- 

 merated 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 rela- 

 tion 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 Am- 

 phistegina (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 meta- 

 morphic 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 ex- 

 tremely 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 



* It is questioned by Messrs. Parker and Rupert Jones whether this is a true Am- 

 phistegina, chiefly on account of its bilateral symmetry (Ann. of Nat. Hist., Feb., 1860, 

 p. 111). But I have met with perfect bilateral symmetry in specimens warranted as 

 Amphistegina by those excellent judges of that type. 



f I have not yet been able to satisfy myself as to the precise affinities of Fusulina, 

 the metamorphic condition of its shell interfering with the minute study of its structure; 

 but my view of its nature essentially corresponds with that of Messrs. Parker and Rupert 

 Jones. (See Quart. Journ. of Geol. Soc, Nov., 1860, p. 458.) 



