232 



KOHKKT TRACY JACKSON ON ECHINI. 



As shown in the diagram by connecting dotted lines, each type in the development of its 

 ambulacrum presents a series of stages, either ventrally as youthful or dorsally as localized 

 stages or both, which are directly comparable to the adult (mid-zone) character of all the simpler 

 preceding types of the family. As the result, a complete genealogical history of the family 

 can be read from the study of the ambulacrum. It is also seen how completely developing 

 plates as localized stages repeat the characters seen as stages in the developing individual 

 gathered from the plates from the ventral border up to the mid-zone. This is comparable to 

 the developing characters occurring in successively added septa in ammonites, as shown by 

 Hyatt and others, where also localized stages occur, as I showed (Jackson, 1899, p. 134). 



The family Lepidesthidae is enlarged over what it was when I first published it (1896) 

 to include certain old genera and one new one. It presents a wide range of characters, yet it 

 is difficult with present knowledge to make two distinct families for the included genera. The 

 sum of the characters affiliates the group as an offshoot from the Palaeechinidae, and the 

 extreme characters seen in several species mark this as one of the most specialized of all groups 

 of Echini. There are from two to twenty columns of plates in each ambulacral area and from 

 three to twelve columns of plates in each interambulacral area. The plates are moderately or 

 more usually strongly imbricate, and bear secondary spines with imperforate tubercles only, or in 

 some genera part or all of the interambulacral plates bear also a primary spine with a perforate 

 tubercle. Where the ventral border of the corona is known, the primordial interambulacral 

 plates are in place, but this area is unknown in some of the genera. In Lepidechinus and Lepi- 

 desthes the peristome is covered with ambulacral plates only, and the same condition probably 

 exists in the other genera. Oculars are all insert or may be exsert. Genitals have from one 

 to many pores each, and the periproct is heavily plated. The lantern has the usual Palaeozoic 

 characters, but in one genus the teeth are distally denticulate. The simplest of the Lepides- 

 thidae are so similar to the simplest members of the Palaeechinidae that the Lepidesthidae may 

 well have been derived from that stock (p. 393). 



The genus Lepidechinus Hall makes a close approach to Palaeechinus. It differs from it 

 mainly in that the plates imbricate moderately, and that interambulacral plates bevel over 

 the ambulacrals instead of the ambulacral plates beveling over the interambulacrals, as in 

 Palaeechinus and all of its family. The ambulacral plates are all primaries, nearly uniform in 

 height, or with a slight tendency for alternate plates to become enlarged against the interam- 

 bulacral suture, pore-pairs are nearly or quite uniserial. Interambulacral plates are in 

 four to eight columns in each area. These plates are rounded hexagonal, with curved surfaces 

 bearing secondary spines and tubercles only. Oculars are all exsert and genitals with a single 

 pore each as far as preserved in L. tessellatus, the only species of the genus in which they are 

 known (Plate 63, figs. 7, 8). The species are all known from fairly complete specimens 

 (Plate 62, figs. 1-5; Plate 63; Plate 64, fig. 1), (p. 394). 



