214 DR. CARPENTER'S RESEARCHES ON THE FORAMINIFERA. 



V 



to exist in regard to superficial area ; and which also proves that the relative thick- 

 ness of the central and of the peripheral portions is equally liable to variation. 



46. It seems obvious, from the foregoing considerations, that neither the absolute 

 nor the relative dimensions of the individual parts of these composite fabrics, can, 

 anymore than the dimensions of the entire disks, be taken as affording valid charac- 

 ters for the discrimination of species ; and that such a wide range of variation exists 

 among individuals, as would, if the extreme cases alone were known, seem fully 

 to justify their separation under distinct specific designations. Thus, if the two 

 extreme forms, figs. 1 and 4 (Plate VII.), had been the sole objects of comparison, 

 most naturalists would undoubtedly have considered the strongly-marked difference 

 in the size of their respective nuclei to entitle them to rank as separate species; and 

 even if fig. 2 or fig. 3 had been brought into comparison with them, it might have 

 been a question whether it should be associated with fig. 1 or with fig. 4, or should 

 rank as a third species intermediate between them, or should be considered as a con- 

 necting link specifically identical with both, and therefore establishing their specific 

 identity with each other. The comparison of the entire series must be felt to remove 

 all ground for hesitation on this point, since it is manifestly impossible to draw a line 

 across any part of it, which should divide it into two or any larger number of groups, 

 respectively characterized by constant and well-marked differences in size. And 

 here again, therefore, we have evidence of the great importance of bringing into 

 comparison a sufficiently large number of forms, to enable us to determine in some 

 degree the measure of individual variation. 



47. Variations in Shape. The very strong tendency which we have seen to pre- 

 vail in the Orbitolite, not only to the maintenance of the circular type in the regular 

 growth of the disk, but to its reproduction after accidental injuries, seems to prevent 

 the occurrence of any considerable variation in its general form, except such as may 

 be directly produced by external agencies. The circular sometimes gives place to an 

 elliptical shape (Plate VII. fig. 4), especially in young specimens, whose form is more 

 determined by that of the nucleus than is that of older individuals. And the occur- 

 rence of such a variety in the recent type, makes me indisposed to admit that ellip- 

 ticity of shape can be in itself a sufficient basis for the specific differentiation of any 

 fossil form of this genus. Of the marked differences in the general aspect of the 

 disks, which may arise from differences in the relative proportions between their 

 thickness and their diameter, and in the relative thickness of their central and 

 peripheral portions, mention has been already made (^[^j 17 and 32.). And the 

 only other important departure from the typical shape which I have met with, either 

 in the simple or in the more complex form, has been described under a former 

 head (f 25.). 



48. Variations in the Form of the Superficial Cells, and in the Markings of the Sur- 

 face. The appearances presented on minute observation by the surface of the Orbi- 

 tolite, are so far from being uniform, that to any one whose eye had not become 



