Schuchert — Jackson on the Phylogeny of the Echini. 255 



intermediate grades representing every step between this least 

 and greatest specialization of the area, and it is a matter of 

 great interest to follow the progressive series as represented by 

 stages in development, and by ad nit types, to see how the pro- 

 gressive differential structure is built up. As the plates of the 

 ventral border are the oldest or first formed of any plates seen 

 in an individual specimen, and as the later added plates 

 succeed one another as we pass dorsally, it might be thought 

 that we could read stages in development as expressed by rows 

 and columns of plates with ease and certainty, and snch can be 

 done in many types . . . Complications may come in, 

 however, especially resorption of the base of the corona by 

 encroachment of the peristome cutting off part of the ventral 

 plates, and also rarely resorption within the corona, as excep- 

 tionally in Arachnoides, or differential growth of associated 

 plates, which may separate plates originally in contact (Echin- 

 arachnius)" (62-4). 



Base of the corona. — " The characters of basicoronal inter- 

 ambulacral plates are the more striking and may be stated in 

 brief. Where no plates have been removed by resorption, 

 there is a single plate at the ventral border of the interambn- 

 lacrum. The primitive type of this character is Bothrioci- 

 daris, which continues to build a single column. This same 

 character of a single plate ventrally, but succeeded by two 

 plates in the second row, is characteristic of the young of all 

 modern regular Echini. ... In the adult of most regular 

 Echini the single plate and probably more have been resorbed 

 by the advance of the peristome (Eucidaris). In the Falae 

 echinidae with many columns of plates, apparently only one 

 plate has been resorbed, when we find two plates in the 

 basicoronal row, ... or in the Archaeocidaridae, several 

 rows of plates may have been resorbed, and we find four plates 

 in the basicoronal row. . . . 



" In Bothriocidaris the basicoronal row consists of two high 

 hexagonal ambulacral plates with pores superposed in each 

 ambulacral area and one interambulacral plate in each interam- 

 bulacral area. This same character is seen in young cidarids, 

 young Strongylocentrotus, and Echinus, young Salenia, 

 Arbacia, and Phormosoma. It is, I think, fair to call this a 

 primitive character, and it represents what I (1896) described 

 as the protechinus stage. The protechinus stage is comparable 

 in other groups of animals to the protoconch of cephalous 

 Mollusca, what I (1890) described as the prodissoconch of 

 Pelecypoda, and to Beecher's (1901) protegulum of Brachio- 

 poda and protaspis of Trilobita. All are referable to what I 

 termed (1890) the phylembryonic stage in development, a 

 stage in which the differential characters of the class are estab- 

 lished in ontogeny" (69-71). 



