CHARACTERS OF BASICORONAL PLATES. 71 



troids and spatangoids (text-figs. 27-29). In Plexechinus (text-fig. 27) the .single plate is 

 followed by another single plate, and these by a second column. This is a very rare exception, 

 as the typical character is for the initial primordial plate to be immediately succeeded by two 

 plates in all Echini except Bothriocidaris. In Rotula (text-fig. 29) the primordial interam- 

 bulacral plate is separated from the next two of its area by the lateral introvenient ambulacral 

 plates. As shown by Loven, plates originally in contact in the young may be thus separated 

 during growth. A few modern regular Echini retain the primordial plate in the adult (Phor- 

 mosoma, text-fig. 43, p. 80; Arbacia, text-fig. 227, p. 193). 



In Palaeozoic Echini with imbricate plates and many interambulacral columns, there is 

 no resorption, and therefore the primordial interambulacral plate is retained in the adult, as 

 in Perischodomus (text-fig. 30), Hyattechinus (Plate 25, fig. 1), Lepidocentrus, Lepidesthes, 

 and Pholidocidaris. The primordial plate in all these is succeeded by two plates, then three, 

 then four, etc. (text-fig. 30). In the remarkable Tiarechinus from the Trias, Loven (1883) 

 showed that the primordial interambulacral plate is succeeded by three plates (text-fig. 31), 

 and then no more plates are built, a unique condition. 



Considering the general characters of the basicoronal row, with special reference to the 

 ambulacral plates, we find the following types of arrangement. In Bothriocidaris the basi- 

 coronal row consists of two high hexagonal ambulacral plates with pores superposed in each 

 ambulacral area and one interambulacral plate in each interambulacral area (Plate 1, fig. 1). 

 This same character is seen in young cidarids (Plate 2, fig. 2) (Loven, 1892; A. Agassiz, 1904), 

 young Strongylocentrotus (Plate 3, fig. 11), and Echinus (Loven, 1892), young Salenia, Arbacia, 

 and Phormosoma (A. Agassiz, 1904). It is, I think, fair to call this a primitive character, and 

 it represents what I (1896, p. 235) 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, p. 289) the 

 phylenibryonic stage in development, a stage in which the differential characters of the class 

 are established in ontogeny. 



A second type of basicoronal plates, in which there are two ambulacral plates alternating 

 with one interambulacral plate in each area, is seen in types where the ambulacral plates have 

 in part flowed down on to the peristome, so that the plates originally there are gone, and yet 

 the primordial interambulacral plate has retained its original place. Such is seen in the Palaeo- 

 zoic Hyattechinus (Plate 23, fig. 1), Pholidechinus (Plate 28, fig. 1), Lepidesthes (Plate 68, 

 fig. 3), and Recent Phormosoma (text-fig. 43, p. 80). This last type differs markedly from 

 those just considered in that the ambulacral basicoronal plates in the adult are compound 

 instead of simple. No compound ambulacral plates on the peristome are known in any type. 



A third type of basicoronal plates is seen in clypeastroids (Echinarachnius, text-fig. 52, 



