PROTEROCIDARIS. 409 



r 



rounded on the suture lines, which I thinlv indicates that the plates were more or less imbricating, 

 a family character. However, as the plates are represented only by external molds, it is diffi- 

 cult to be certain on this point. Adambulacral plates are pentagonal, median plates hexag- 

 onal or an equivalent of the same. Part, but not all, of the adambulacral plates bear a 

 large, elevated, perforate primary tubercle which is the largest known in the family, and is 

 surpassed in size in the Palaeozoic only by the Archaeocidaridae. A large tubercle is on the 

 second and fourth adambulacral plates from the apical disc (Plate 67, fig. 2), and this peculiar 

 system seems to hold for each area, as pointed out by Harte and by Baily (1874). The latter 

 also shows numerous secondary tubercles scattered over all the interambulacral plates, and, 

 according to his figures there are primary tubercles on some additional adambulacral plates, 

 also on a few plates of the median columns. 



The apical disc is in place complete (Plate 67, fig. 3). The oculars are small and imper- 

 forate, and all separate the genitals, meeting the periproct as in Melonechinus, etc. Ventrally, 

 they cover the ambulacra and laterally the interambulacra in part on either side. The genitals 

 are high and wide, almost V-shaped on their adoral termination. Four of the genitals have a 

 prominent primary tubercle, but it is wanting in one, very likely from imperfection of the mold. 

 This is the only case of large primary tubercles on genital plates known in the Palaeozoic, 

 and it is exceptional in any Echini. None of the genitals has sixteen pores, the number given 

 by Harte and Neumayr. Baily pointed out this error and said that there were six pores in 

 four of the genitals arranged in a semicircle, ventral to the large tubercle. In my observations 

 I found six pores in genital C (Plate 67, fig. 3), five pores in genitals A and I, and only three pores 

 in genital G. In genital E no pores were seen, but this may fairly be ascribed to imperfection 

 of the sandstone mold, not to any structural difference of this plate. A similar apparent ab- 

 sence of pores is seen in several genital plates in Plate 53, fig. 1. Harte and Sollas consider 

 the genital without primary tubercle or pores as a madreporite. This may be correct, but in 

 the absence of madreporic pores, which are very rare in the Palaeozoic (p. 172), this view must 

 be taken with qualification. 



This is one of the most distinct species of Palaeozoic Echini, making no close approach 

 to any other known form. It is from the Lower Carboniferous, yellowish sandstone, Lough 

 Esk, about six miles from Donegal, Ireland. This important and unique specimen is in the 

 collection of Trinity College, Dublin. 



PROTEROCIDARIS Koniuck (emended). 



I'rotcruridaris Koninck, 1881, p. .514, Plate S. 

 OUgoporus (pars) Lambert and Tliiery, 1910, p. 121. 



In describing this genus, de Koninck did not give the characters of the ambulacrum, but 

 his figure of the type and only known species, Proterocidaris giganteus de Koninck, bears a close 



