576 DR. CARPENTER'S RESEARCHES ON THE FORAMINIFERA. 



Hence the analogy of Operculina affords good ground to surmise that many of the 

 reputed species in the nearly-allied genus Nummulites have no real title to that rank ; 

 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 (^[ 162) that it may be fairly questioned whether there 

 is adequate ground for upholding the generic distinctness of Operculina and Nummulites ; 

 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 Cuminyii* 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. 



247. There appears, then, strong reason for considering Cycloclypeus, Heterostegina, 

 Operculina, Nummulites, and Ampliistegina 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 just pointed out, the relation of Cycloclypeus to 

 Heterostegina is exactly that of Orbitolitcs to OrMculina. 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 Orliculina gives to it all the essential characters 

 of Peneroplis. And the parallelism seems to be completed by the existence in Fusulina^ 

 of the same elongated condition of this type, that Alveolina is of the Orbiculine. 



248. The mutual 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 of two distinct laminae, which 

 diverge from each other where they give passage to the canal-system, and which are 

 often further separated by the intervention of a portion of the " intermediate skeleton." 

 The passages of communication between the chambers are so narrow, that the segments 



* It is questioned by Messrs. PAEKER and RUPEET JONES whether this is a true Ampliistegina, chiefly on 

 account of its bilateral symmetry (Ann. of Nat. Hist. Feb. 1860, p. 111). But 1 have met with perfect 

 bilateral symmetry in specimens warranted as Amphistegince by those excellent judges of that type. 



t I have not yet been able to satisfy myself as to the precise affinities of Fusulina, the metamorphic con- 

 dition of its shell interfering with the minute study of its structure ; but my view of its nature essentially 

 corresponds with that of Messrs. PAEKEE and RUFEET JONES. (Sec Quart. Journ. of Geol. Soc , Nov. 1860, 

 p. 458.) 



