206 R. T. JACKSON — STUDIES OF PAL.EECHINOIDEA. 



6, figure 25) ; they are also the equivalents of a-\-a' and h-\-U in Oligo- 

 porxis (laiipe (plate 6, figure 30) and Melonites multiporus (plate 2, figure 4, 

 and plate 5, figure 20). 



The interanibulacral plates of Rhoechinus elegans have their edges in- 

 clined slightly outward, and the adambulacral plates extend under the 

 sides of the adjacent ambulacrals, as in Melonites (plate 2, figure 5) and 

 Oligoporus. This feature is doubtless characteristic of the family, as it is 

 existent in these representative genera. 



The method of plate arrangement and introduction in the interambu- 

 lacrum of Palasechinus as shown above is seen to conform to the laws of 

 growth that have already been demonstrated in Melonites and Oligoporus 

 in the earlier part of this and the preceding paper. 



J Studies of the Lepidesthid.e, fam. nov. 



RELATIONSHIPS AND CHARACTERISTICS. 



The family Lepidocentridaj is composed of genera associated as nearest 

 allies with the family Archicocidaridffi, but separated by strongly imbri- 

 cating ])lates and other features. In the genera LcpUlcxthes and Pholi- 

 docidaris we have another group of types also characterized by imbricat- 

 ing plates. The species of these genera all have 6 or more columns of 

 ambulacral jDlates in each area, and this feature allies them with the 

 Melonitida3. In fact, they have always been included in this family. 

 These genera, however, have characters wliich distinguish them as a 

 group by themselves, and a separate fixmily is therefore founded for their 

 reception. 



The Lepidesthida?, fam. nov., is named from Lepidesthcs, which genus 

 is the least aberrant and is rej^resented b}^ the largest number of species of 

 either included genus. The family is characterized (see SN'^stematic class- 

 ification facing page 242) by having from 6 to 10, and in one species 18 or 

 perhaps 20, columns of ambulacral plates in each area. The interanibu- 

 lacral areas of the 2 genera and several species have from 3 to 6 columns 

 of plates, though this number is likely to be increased l)y more perfect 

 specimens or perhaps new species. The peristome is small with no in- 

 terambulacral plates resorbed, as known in Pholidocidaris (figure 54, 

 plate 9) from observation, and known in Lcpidcstlies from ^leek and 

 Worthen's description of L. corcyi.^ One unusual feature is characteristic 

 of the genera of this family, that the pores occupy a position in the center 



♦This character is peculiar to several aberrant groups of Echini (as discussed later on page 

 237), being a feature of the Lepidocentridaj (Lepidcchinus, plate 7, figure 42) and tlie Exocyclioa 

 as well as the present family. It is characteristic of the young of all Echini, and therefore is to be 

 considered a primitive character and probably retrogressive in aberrant types. 



