5G8 



DR. W. B. CARPENTER ON ORBITOLITES TENUISSIMA. 



as is at present known, any tendency to pass into a higher form. Indeed, the 

 typical forms of the existing 0. complanata 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. VIT. 



cation, it can scarcely be supposed that every one of them is a " potential " 0. tenuis- 

 sima. So, again, as we find O. marginalia 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. For 

 I have never observed in the largest and best developed examples of 0. marginalis 

 and O. 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 



