DR. W. B. CARPENTER ON ORBITOLITES TENUISSIMA. 
568 
as is at present known, any tendency to pass into a higher form. Indeed, the 
typical forms of the existing 0 . complcincita are in one small particular (as already 
shown, p. 565) more specialised than the fossil forms that were so remarkably abund¬ 
ant in the Middle Tertiary epoch. 
It is a remarkable feature of this case, however, that all the forms through which 
the highest Orbitoline type is thus shown to have passed, continue to hold their 
ground at the present time, as the characteristic representatives of less specialised 
groups. There being every reason to regard Cornuspirce, Peneroplides, and Orbiculince 
as distinct races, propagating themselves genetically without any essential modifi- 
Fig. VII. 
cation, it can scarcely be supposed that every one of them is a. “ potential 0. tenuis- 
sima. So, again, as we find 0 . mcirginalis and 0 . duplex living and propagating 
under the very same conditions as 0 . complanata, I cannot regard these “ simple 
forms of the Orbitoline type, each of which has its characteristic plan of structure 
and limit of growth, as potentially “ complex; ” notwithstanding the exact repetition 
of their plans in the early stages of certain examples of the higher type. Foi 
I have never observed in the largest and best developed examples of 0 . marginalis 
and 0 . duplex the least tendency to assume the “ complex form ; on the other hand, 
I have frequently found their last formed annuli deficient in internal partitions, as if 
their productive power had exhausted itself. It would seem, therefore, more just to 
