300 Grdbau—Biseynal Arm in Certcdn Grinoids. 



remain in that state, tlie later added ones passing beyond this 

 stage into a cuneate condition. In still more specialized types 

 the later added plates are endowed with potential biseriality, 

 which appears in the development after the youthful condition 

 is passed. In accelerated types earlier and earlier plates are 

 thus endowed, the biserial condition appearing in them in the 

 inverse order of their age, until only the oldest one or two 

 remain uniserial. It is to be expected that in old age indivi- 

 duals this endowment with potential biseriality is weakened, 

 or perhaps absent altogether, in which case gerontic individuals 

 would permanently remain uniserial. From this we might 

 argue that phylogerontic types would have permanently uni- 

 serial plates in the normal adult ; and thus, carrying the argument 

 to its logical conclusion, we may have in extreme phylogerontic 

 types uniserial plates in the greater part or the whole of the 

 arm, even though biserial conditions obtained in ancestral 

 types. Phylogerontic types may have biserial conditions 

 begin very early in the adult individual, while at the same 

 time a large number of uniserial plates are found at the sum- 

 mit of the arm. Dichoerinus inornatus (figs. 9 and 10) may 

 possibly serve as an illustration of this. On the other hand, we 

 may expect primitive types to have a large number of uniserial 

 plates at the base of the arm in the adult, while at the tip of 

 the arm only a few uniserial plates would exist at any time. 



In adults of acmic types only a few uniserial j^lates, or none 

 at all, should exist at the base of the arm, while at the summit, 

 likewise, very few uniserial plates are to be looked for. In 

 fact, it is not difficult to conceive that in highly accelerated 

 types the newly introduced plate may be cuneate if not biserial 

 from the beginning. For examples of this we must look 

 among the highly accelerated Actinocrinidge and Batocrinidse. 



While a uniserial apex in a biserial arm may represent old 

 age conditions in the individual or the race, as well as a patho- 

 logic state, it does not always represent this. For uniserial 

 apical conditions have been found in individuals of all ages, 

 even young ones, so that it is perfectly clear that normally a 

 uniserial character of an apical plate indicates the youth of 

 that plate, and only secondarily the old age conditions of the 

 individual. 



Paleontological Laboratory, 

 Columbia University. 



