oi;m:n in 



PAL.KKCmXOIDKA I'KIUSCHOKCIIINOIDA 



221 



FIG. 353. 



Palceechinus elegans, M'Coy. Carboniferous Limestone ; Ireland. 

 A, Test, Vi- -B> Apical system, enlarged (after Baily). 



Family 2. Melonitidae. Zittel. 1 



Ambulacra more or less broad, composed of several columns of poriferous plates 

 in all genera except Palceechinus ami Ehoechinus, where there are but two. Car- 

 boniferous. 



Palceechinus (Scouler), M'Coy (Fig. 353). Test spheroidal, rigid. Amb 

 straight, composed of two vertical rows of low thick plates, extending 

 across the half Amh or not. 

 Pairs of pores in two ver- 

 tical rows on each side of 

 an Amb; the outer pairs 

 either in demi- plates or 

 primaries; the inner pairs 

 always in primaries, which 

 may, however, be short. 

 lAmb broad, convex, with 

 five to eight columns of 

 tumid plates, diminishing 

 toward the poles. Peri- 

 proct in apical system, sur- 

 rounded by five large basal 

 plates, each perforated by 

 one or three canals, and five doubly perforated radial plates. Silurian ; 

 England. Carboniferous Limestone ; Great Britain. 



PJwechinus, Keeping. Like the preceding, but ambulacral plates never 

 compound, and only one vertical row of pore pairs on each side of an Amb, a 

 pair to each plate. lAmb with four to eight columns of plates diminishing 

 toward the poles, slightly overlapping. Radials sometimes wanting. Car- 

 boniferous Limestone ; Great Britain. Sub-Carboniferous ; North America. 



Oligoporus, Meek and Worthen. Like Melonites, but Amb with only four 

 columns of plates, and lAmb with five to nine columns at the ambitus, diminishing 

 in number toward the poles. According to Jackson, the interambulacral plates 

 develop like those of Melonites. The Amb terminate at the peristomial margin 

 in two plates, which pass gradually by lateral expansion into the four columns 

 of the adult. Sub-Carboniferous ; North America. 



Melonites, Norwood and Owen (Fig. 354). Test very large, ellipsoidal, 

 grooved longitudinally. Amb broad, concave on both sides of a median ridge, 

 with six to twelve columns of plates, each perforated near its outer border by a 

 pair of pores. Plates slightly imbricated, the median rows the largest. lAmb 

 with four to eleven columns of plates, diminishing in number toward the poles. 

 The median plates are hexagonal ; the two rows adjacent to the Amb pentagonal, 

 with the edges indented by the zigzag of the ambulacro-interradial suture. 

 Edges of plates sometimes oblique, especially when thick. Tubercles very 



1 Roemer, F., Ueber den Bau von Melonites multipora (Arch, fur Naturgesch. Bel. XXI.), 1855. 

 EtJieridge, R., On Relationships between the Echinothuridae and Perischoechinidae (Quar. Journ. 



QeoL Soc. vol. XXX.), 1874. 



Keeping, IV., On the Discovery of Melonites in Britain (Quar. Journ. Geol. Soc. vol. XXXII.), 1876. 

 Duncan, P. M., On the Anatomy of Palseechinus (Ann. and Mag. Nat. Hist. [6] vol. III.), 1889. 

 Keyes, G. R., Palaeozoic Echiuodermata (Proc. Iowa Acad. Sci. vol. II.), 1894. 

 Jackson, R. T., and Jaggar, T. A., Studies of Melouites multiporus (Bull. Geol. Soc. America, 



VII.), 1896. 

 Jackson, R. T., Studies of Palseechinoidea (Bull. Geol. Soc. America, VII.), 1896. 



