SYSTEMATIC CLASSIFICATION OF ECHINI. 223 



composed of many rather thick plates close to the character of the periproct in the Cidaroida. 

 The lantern is known well for the Palaeozoic onlj' in this order, and is strikingly uniform in 

 character. Mr. Agassiz (1881, p. 80) suggested and Duncan (1889a, p. 11) stated that there 

 are passages for peristomal gills in Archaeocidaris, but such have not been figured. As there 

 was no indication of such slits in the best specimens which I studied (Plate 9, fig. 6 and Plate 

 11, fig. 1) and as such structures are otherwise known only in the Centrechinoida and Holec- 

 typina, I think this is a mistake, and peristomal gills may be considered as wanting in 

 Archaeocidaris and other Palaeozoic genera as well (p. 253). 



The Archaeocidaridae is a family with only three genera, Eocidaris, Archaeocidaris, and 

 Lepidocidaris. Eliminating Eocidaris, which is most imperfectly known, there are two columns 

 of ambulacral plates and four, or six to eight of interambulacral plates in each area respectively. 

 The plates are rather strongly imbricate, and each interambulacral plate bears a central pri- 

 mary perforate tubercle and large spine, also secondary spines and tubercles. The base of the 

 corona is extensively resorbed, there being four plates in the basicoronal row in each interambu- 

 lacral area. The peristome has many rows of ambulacral plates with non-ambulacral plates 

 in Ai'chaeocidaris (Plate 9, figs. 6, 7) and probably was similar though unknown in Lepido- 

 cidaris. The apical system is doubtfully known, which is rather remarkable with so many 

 species. The lantern is well known in Archaeocidaris (Plate 12) and consists of forty pieces 

 as in modern regular Echini (p. 254). 



The genus Eocidaris has large interambulacral plates which bear a central perforate 

 tubercle, as in Archaeocidaris; there is a scrobicular area but no basal terrace. This genus is 

 most imperfectly known, but Bather (1909), who has studied the question and material critically, 

 feels that it should be retained. Only one species is recognized, the Devonian E. laevispina 

 (Sandberger) , Plate 15, figs. 11, 12. The absence of a basal terrace, which is the essential 

 difference from Archaeocidaris, could easily be accounted for by wear as it is not always ob- 

 servable in true Archaeocidaris. The young dorsal plates of Archaeocidaris have no basal 

 terrace (Plate 11, fig. 2), so that a genus founded on this character should be considered the 

 more primitive of the two (p. 254). 



In Archaeocidaris the ambulacral plates are narrow and low, with pore-pairs uniserial 

 (Plate 12, fig. 9). In the few species where the corona is fairly known, there are four columns 

 of large plates in each interambulacral area (Plate 10, fig. 10). Each plate bears a large central 

 perforate tubercle with a basal terrace and scrobicular ring (Plate 11, fig. 4). The basal terrace 

 is so slight an eminence that it is often worn off, and this feature can only be ascertained in 

 fairly well preserved specimens. From the center rises a primary spine of considerable size 

 and various ornamentation; in addition, secondary and miliary spines and tubercles occur 

 on the periphery of the plates. The young plates dorsally (Plate 11, fig. 2) are at first smooth, 

 then an imperforate tubercle arises (as in developing plates of cidarids, Plate 3, figs. 1, 2) 



