RESORPTION AND IMBRICATION OF PLATES. 237 



•either no or very slight resorption of the corona, and this is the more 

 striking as the several groups are widely separated. These groups are 

 characterized by imbricating plates (except TiarecJdnus, in which plates 

 are not imbricated). The groups of Palseechini which have no or ver}^ 

 slight resorption of the corona are the Cystocidaroida (Echinocystites, 

 figured by Thomson (39)), the Plesiocidaroida (Tiarechinus, see figures 

 by Loven (26)), the Lepidocentridse (Lepidechinus, plate?, figure 42) and 

 the Lepidesthidae (Pholidocidaris, plate 9, figure 54) ; compare with table 

 facing page 242. These several groups of echinoids are irregular in being 

 specialized in several aberrant characters as compared wdth their nearest 

 normal allies. The group of the Paleozoic Echini, therefore, seems to 

 present 3 natural divisions as regards relations to the peristome : primi- 

 tive, with no resorption (Bothriocidaris') ; progressive, with more or less 

 resorption, and aberrant, with no or very slight resorption* (see page 144). 



Imbrication of coronal plates is a feature which appears as a character 

 in several groups of Paleozoic echinoids. It is seen well developed in 

 the Lepidocentridse, the Lepidesthidse, and the Cystocidaroida among 

 the Palseechinoidea.f Appearing in such widely separated types, it ap- 

 pears that imbrication is a variation built up on independent lines. 



All the genera of the Palseechinoidea, as far as known, start with a 

 single plate at the ventral border of the interambulacrum, and above 

 Bothriocidaris all increase to several (Tiarechinus) or many columns in 

 the adult. None of these types show evidence of a return from a many 

 •column to a few column type except in so far as indicated in the early 

 dropping out of the fourth column in Lepidesthes wortheni (plate 9, figure 

 53), and as slightly indicated in the dropping out of the eleventh column 

 near the dorsal border in Melonites giganteus (plate 5, figure 21), and the 

 brief period of existence of the sixth column in Oligoporous missouriensis 

 (plate 9, figure 50). They all, therefore, apparently culminate in these 

 extreme forms. 



The Euechinoidea as representedby Cldaris (page 234), in the develop- 

 ment of the corona show strong evidence in both ambulacral and inter- 

 ambulacral areas, pointing toward Bothriocidaris as a parent form. In the 

 Upper Silurian and Devonian Ave know only a few aberrant types, so that 

 we must suppose that intermediate forms between the primitive Bothrioci- 

 daris and the several derivative types are at present quite unknown. 

 Tetracidaris in the Cretaceous has 4 columns of plates in the lower part 



* Of the Exocyelica in the subclass Euechinoidea we have no Paleozoic representatives. This 

 group as a whole is characterized by retaining in the adult the single initial plates of the interam- 

 bulacral areas, as in aberrant types of Palseechinoidea, see numerous figures by A. Agassiz (3), 

 Loven (25), Clark (6) and others. In this group, the Exocyelica, also we have specialized features, 

 as bilateral symmetry, an eccentric periproct and peculiarities in the form and arrangement of 

 plates and spines not seen in regular forms (Endocyclica) of its subclass. 



t Imbrication is also a feature of the Cretaceous and Recent family, the Ecliinothuridse in the 

 Euechinoidea. 



