VALIK AM) rSE OF STAGES IN DEVELOPMENT 397 



ilar to the structure in adult Aulopora. Points of possible budding, 

 where buds did not develop tlirough nieclianical lack of space, are indi- 

 cated by the pores in the outer wall of the corallites of Favosites. 



In his extensive studies of fossil and recent Brachiopoda, Beecher 

 showed that from the development of tlie several groups we could gather 

 a full understanding of the origin and structure of the shell from its 

 early protegulum stage. He compared the protegulum with the adult of 

 Paterina, from the Cambrian, as an archaic radicle. He elucidated 

 greatly the structures of the deltidium, deltidial plates, and calcareous 

 supports of the arms from their developmental stages, and, moreover, 

 built up a satisfactory classification of the class, Brachiopoda, based on 

 the ontogeny of representative types. The shell of the brachiopod being 

 an external structure, much can be gathered as regards later stages from 

 studying the lines of growth of the test, unless they have been destroyed 

 l)y mechanical wear, or resorption in the enlargement of the pedicle 

 opening. 



Paleozoic Echini, with their numerous columns of interambulacral 

 plates and often numerous columns of ambulacral plates as well, have 

 been difficult to bring into accord Avith modern Echini, which have but 

 two columns in each of the said areas. Jackson has definitely shown 

 that, from a study of the test, passing from the ventral border dorsally 

 and including a study of the young as well, new columns of plates 

 are added progressively in the interambulacrum from one column ven- 

 trally (represented by only a single plate in all but Bothriocidaris) to 

 two or more, up to fourteen columns dorsally, according to the type in 

 hand. In the ambulacrum with two columns ventrally as the primitive 

 condition, passing dorsally they may increase to four, six, etcetera, up to 

 twenty columns of plates in an area in extreme forms. These changes 

 in paleozoic Echini (when resorption has not removed any plates ven- 

 trally) may even be observed on a single well preserved specimen by 

 studying the zones of plates from the ventral portion of the test and 

 passing progressively dorsally to the mid-zone and finally to the apical 

 disk, in which zone the new, last added plates appear. 



The stages in development of both living and fossil Echini elucidate 

 the structure of the adult and point to Bothriocidaris of the Ordovician, 

 with a single column of plates in each interambulacrum and two columns 

 of high plates in each ambulacrum as a primitive archaic radicle. The 

 development of the ambulacrum in tlie Palaechinidse demonstrates the 

 genetic relations of the genera in that large family. Throughout the 

 Echini stages were the main basis of an attempted natural classification 

 of the group. 



