266 FAMILY NUMMULINIDA. 



these, if not of all ; but the knowledge since acquired of the range of variation among Fora- 

 minifera generally, and in the allied genus OpercuUna in particular, justifies a doubt whether 

 the diiferences between them are really more than varietal, and whether, if a sufficient number 

 of individuals -were compared, they would not be found as gradational as are those wc have met 

 with elsewhere (see ^^ 434 — 438). The latest contribution to our knowledge of the structure 

 of Nummulites is contained in a recent paper by Mr. Carter (xxiii a), which contains the 

 results of his further study of the subject, especially in regard to the distribution of the canal- 

 system. I feel it due to myself to state that the account of that system which I shall pre- 

 sently give is entirely based on my own previous re-investigation of it, with the light which 

 I have derived from my examination of Opercullna ; and that the illustrations in Plate XVIII 

 had all been not only drawn but printed before the appearance of Mr. Carter's paper, 

 in which as to most points it will be found that I fully concur. In regard to the question of 

 specific distinctions in this genus, I have been led by my own independent observations to an 

 entire concurrence with the views recently put forth by Messrs. Parker and Rupert Jones 

 (lxxx h), whose very careful and thorough investigation of the subject entitles their views to 

 the respectful consideration of such as have formed their notions of the differentiation of 

 species on the ordinary descriptions based on the examination of small numbers of specimens. 



456. External Characters. — What may be considered the typical or characteristic form 

 of the Nummulite, is a double-convex lens of moderate thickness ; but the convexity gives 

 place in certain cases to a flattening which approaches that of the flattest varieties of Oj^er- 

 ctdina, whilst in other instances it is so greath' augmented that the lenticular form is con- 

 verted into the spheroidal. A wide range of variation exists in this respect among indi- 

 viduals of what must be accounted (according to the views of the best systematists) one and 

 the same species. Thus, in a Nummulite very common in Scinde (which was formerly con- 

 sidered by INIr. Carter to be the N. obtusa of Sowerby, but which he now regards as the N. 

 perforata of D'Orbigny), the thickness of the lenticular specimens is a little less than half the 

 diameter, the former being 4| lines while the latter is IOt ; but the proportion of the 

 thickness to the diameter may increase until it reaches nearly three-fourths, the former 

 being 9 lines whilst the latter is 1 2\, so that the body becomes globose ; whilst, on the 

 other hand, the thickness may diminish until it is scarcely more than one-third of the 

 diameter, the former being 2\ lines and the latter 7. And what is even more remarkable, in 

 a recent form which I have described (xv) under the name Amphister/ina Cuniingii, but which 

 I am now satisfied, for reasons already given (^ 420, note), to regard as a true Nummidina, 

 although the form of the shell up to neai-ly its full growth is that of a rather thick bi-convex 

 lens, yet the last whorl spreads out in a manner which alters its contour to even a more 

 remarkable degree than we ever see in OpercuUna. The two lateral surfaces have usually a 

 nearly equal curvature ; and any want of s)'mmetry is usually due rather to a departure of 

 the median spiral from the plane on which it normally revolves, than to any excess in the 

 growth of one side beyond that of the other — though such an excess unquestionably does 

 occasionally present itself. The diameter of Nummulites usually ranges between 1-I6th inch 



three, 2 in four, 3 in five, 1 in siie, 1 in seven, and 1 in eigtit different genera ; and of specific names, 

 4 species had received tliree, 1 four, 3 five, 2 six, 1 seven, 1 nine, 2 ten, and 1 eleven. 



