98 INVERTEBRATE PALAEONTOLOGY 



evolution. Beecher proved its value in great numbers 

 of Brachiopods and Trilobites, while many authors have 

 applied it to the study of Ammonites, Gastropods and 

 Pelecypods. Faunal succession is available to confirm 

 or disentangle the records of post-larval ontogeny. 



In several Invertebrate groups, notably the Echino- 

 dermata, new structures which are practically repetitions 

 of those already present are developed throughout 

 life. In Crinoids, the stem is lengthened by addition 

 of new columnals ; in Echinoids, the corona increases 

 in size partly by introduction of fresh plates around 

 the margin of the apical system. Organisms with such 

 modes of growth will always possess some "young" 

 structures, however "old" the individuals may be. 

 For example, the common Holectypoid, Plesiechinus 

 ornatus (PI. vii. fig. i) shows certain coronal plates at 

 the apex that are small, simple, and in course of develop- 

 ment. These morphogenetically neanic ossicles are, 

 in most respects, similar to the corresponding structures 

 produced in early stages of individual ontogeny. From 

 the time of their introduction onwards, such plates 

 progress in growth and elaboration until they attain 

 a degree of specialization harmonizing with the onto- 

 genetic stage of their owner. Study of the inter- 

 ambulacrum of an adult Plesiechinus shows that the 

 plates near the peristome, which date from the early 

 youth of the specimen, are small, and support few 

 tubercles. They represent the slight advance of mor- 

 phogenesis, corresponding with the short progress made 

 in ontogeny at the time when they constituted the 

 whole area. As the columns are traced away from 

 the peristome, the size and tubercular ornamentation 

 of the plates show steady increase, until, at the mid- 

 zone (half-way up the corona, Plate xv.), they exhibit 

 the highest phase of morphological complexity proper 



