MONOGKAPH OF THE EXISTING CRIXOIDS. 211 



The most primitive type of coUimnal has about its center a raised band mark- 

 ing the position of the original aimuUis from which the rest of the columnal has 

 been built up. This band, however, is only preserved m comparatively rare 

 instances, and usually only m the columns of small and deUcate forms, such as ^ 

 Ehizocrinus lofotensis (fig. 135, p. 205). 



The prunitive form of the termuial stem plate is a ch-cular disk (fig. 532, pi. 3), 

 and this is the form first taken in all young pentacrinoids. In some pentacrinoids, 

 and m a few of the stalked species, this form is maintaino.l with little or no varia- 

 tion, but m many pontacrmoids the originally circular disk grows not by a uniform 

 extension of its enthe border, but by more or less definitely locaHzed additions of 

 calcareous matter, so that it becomes lobate or, m extreme cases, sharply digiti- 



form (figs. 533-540, pi. 3). 



The terminal stem plate in Promachocrinus is strongly lobate or more or less 

 di<-itiform, suggestmg that of the species of Eathrometra. This type of stem plate 

 always accompanies greatly elongated columnals m pcntacrmoid larv-ffi. If the 

 columnals are very short the terminal stem plate approaches a cu-cular form length- 

 ening columnals bemg correlated with an mcreasmgly lobate outhne, which finally 



becomes digitiform. • i ^ • i 



Pentacrinoid larvae with short columnals and a more or less cu-cular termmal 

 stem plate in other words, wdth a column of comparatively slow growth, never 

 show any trace of radicular chri; but pcntacrmoid larvae with veiy long columnals 

 and a strongly digitiform termmal stem plate, that is, ^^-ith a verj^ rapid stem 

 growth, often form additional attachments further up the column (figs. 540, 



541, pi. 3). , , t • ■ re 1.1-7 



Radicular cirri are entirely distmct from the other type of cim (fig. 127, 

 T> 197)- they are most perfect at the base of the column and rapidly become 

 smaller' and less perfect toward the crown. The true cirri are always absent 

 from the base of the column, first appearmg, usually in a deficient series of 

 more or less unperfect mdividuals, just beyond (reckonhig from the terminal 

 stem plate) the first stem syzygjs the most perfect and the best developed bemg 



iust under the crown. , , , , ^u ,\. 



The radicular cirri are merely special processes developed from he overgrowth 

 and expansion of the terminal stem plate, and are always confined to the region 

 below t^he first stem svzvg}'; the true cirri represent five dorsal processes, or groups 

 of processes, one from each of the five metameric divisions of the body. 



Radicular cu-ri are probably to be mterpreted as originally a ternunal stem 

 plate which is reduplicated tlu-ough a number of columnals on account of the very 

 rai>id -rowth of the latter; that is, a number of the oariiest co umnals possess a 

 tendency, progressively decreasmg, to expand laterally at the ends; but on accoun 

 of the fulcral ri.lge such expansion can only take place at two pomts, so that it 

 forms two long processes, one on either side. . 



The radicular cirri themselves are best consi.lered as representing a s ep in 

 development bevon.l the .ligitiform type of terminal stem plate; this form of stem 

 plate is developed from the circular through the lobate as a result o a great mcrease 

 L the rate of growth; further hierease in the rate of growth results ui immensely 

 increasing the length of the digitiform processes, which become jointed and branched. 



