244 ROBERT TRACY JACKSON ON ECHINI. 



in Schmidt's specimen 18 mm. high, 18.5 mm. in diameter. Eichwald's type was smaller, only 

 12.5 mm. in diameter. There are two columns of high hexagonal plates in each ambulacra! 

 area, and the plates at the ambitus are 4 mm. wide and 3 mm. high. Interambulacral plates 

 are somewhat narrower, 3 mm. wide. Schmidt says that there are ten ambulacral plates in 

 each column, which, subtracting two for the peristomal rows, leaves eight in a column for the 

 corona, as in the two other species. The pore-pairs are superposed in the middle of each plate, 

 and at the ambitus there are four perforate tubercles around the peripodia. The interambula- 

 cral plates at the ambitus have two or three perforate tubercles, but ventrally and dorsally 

 only one. Schmidt says that the spines attributed to this species by Eichwald have nothing 

 to do with it, but appear to be small Bryozoa. The peristome consists of two rows of ambula- 

 cral plates as in the other species; the dorsal border of the second row is in contact with the 

 ventral border of the primordial interambulacral plates. The oculars are large, the largest 

 of which I take to be III, as in B. archaica. They bear one, two, or five tubercles each, but 

 ocular IV has no tubercle, probably worn off. Four genital plates are in place, but genital 4 

 is absent or displaced (Plate 1, fig. 9). The genitals are larger than in the other species; each 

 bears a tubercle and separates the oculars, therefore reaching the interambulacra as they do 

 in all later Echini, but in this feature differing entirely from archaica and partially from pahleni 

 (p. 240). There are only two periproctal plates in place lying dorsal to oculars II and III and 

 separating the associated genitals. Schmidt figures five minute plates ventral to the primordial 

 ambulacral plates, and which I take to be teeth or dental pyramids, as in the other species 

 (discussion, pp. 181, 242). 



Schmidt's specimen is from the Lyckholm Schicht, Ordovician, Hohenholm, Island of 

 Dago, Russia. The type is from the same locality. 



It is very interesting historically that Aldrovandus in 1618 gave a figure that is recognizable 

 as belonging to Bothriocidaris and probably to this species. The figure is entitled "Echinus 

 lapis spoliatus a spinis," but there is no description. While somewhat rough, the figure shows 

 two columns of high hexagonal ambulacral plates, with a cup-shaped depression in the center 

 of each, and a single column of high hexagonal interambulacral plates which apparently meets 

 the genitals. There are from one to three tubercles on the interambulacral plates; these 

 features apply to this species and no other. It is remarkable that this excessively rare echinoid 

 should have been found so early and then lost sight of until recent years. 



Order CIDAROIDA Duncan. 



Cidaraida Duncan, 1889a. 



Regular Echini with periproct enclosed by oculo-genital ring. Two columns of low, narrow 

 ambulacral plates and two columns of pentagonal interambulacral plates, the latter with a 

 large, central primary tubercle and spine, also marginal secondary tubercles and spines. Other 



