128 ROBERT TRACY JACKSON OX 



tlie cirrlii close to tlie cmIvx are not so al)iiiiilaiit as in ^fifacrij/ns rofiindiix. Tii the 

 Jurassic Pentacrbuis [Extrmrhivs) briareiis Queiist. the stein is pentagonal throughout 

 its length (PI. 25, figs. 125, 125a), and is (len.selj clotiied with eirrhi. Other species of 

 Jurassic Pentacrinus also ha\ e pentagonal stems, as I\ basaftiformis Miller (PI. 25, fig. 

 ]2G). The young last formed pentagonal plates of modern Pentacrinus and Metacrinus 

 throughout life, therefore, hear a close resemblance to the whole stem of their ancient 

 fossil representatives. 



In Flafi/crijujs sipninctrh-iis Wachs. and Spring. (PI. 25, fig. 127), from the Subcar- 

 boniferous, and other species of the genus, the plates of the stem are oval in cross- 

 section, and set on one an(jther in such a manner that the axes of each plate are slightly 

 different from those of the preceding plate. The result is a spirally twisted stem. On 

 examining the young plates close under the calyx, it is seen that they are circular and 

 smaller than the older plates of the stem. During growth, as new plates are added, the 

 round plates grow and take on the oval and spiral form of the older plates of the stem. 

 Whatever the ancestry of Platycriuus, it is probable that it came more or less directly 

 from forms with a simple circular type of stem plate similar to that which is seen in the 

 young plates close up under the calvx. 



In crinoids, as in starfishes, the arms grow by the additiijn of new plates at the 

 distal ends. As a consequence, the plates at the proximal end of the arm were formed 

 first in the ontogeny of the individual and the plates at the distal ends of the arms are 

 the youngest or last added plates. 



In Encrinus lUuforrnlti Mill., of the European Ti'iassic, the plates at the base of the 

 arms are mcjuoserial, or arranged in a single row, each plate extending aci'oss the arm. 

 Passing outward, the arm plates take on a biserial arrangement, the plates extending 

 only half way across the arm; this condition is characteristic of the greater part of the 

 length of the arm. As observed by Mr. A. W. Grabau, the plates at the tips or youngest 

 portion of the arms are monoserial, in tins feature resemljling the oldest or first formed 

 plates at the base of the arms. During the develojMuent of these plates at the tijjs of 

 the arms, as shown by a series of specimens, the plates become relatively shortened, 

 cease to pass entirely across the arm, and take on a biserial arrangement, like the plates 

 on the intermediate portion of the arm. 



Ecuixi. Certain sea-urchhis show localized stages in development in a typical way. 

 In the growth of the sea-urchin newly added plates of the corona are introduced at the 

 dorsal border between the genital or ocular plates and the last formed plates of the 

 ambulacral or interambulacral areas. Therefore the ])lates at the dorsal portion of the 

 corona are the youngest or last added plates, and other plates of the corona are pro- 

 gressively older as we proceed ventrally to the peristome. 



