ORDER III PALHECHINOIDEA—PERISCHOECHINOIDA ip 
bo 
— 
Family 2. Melonitidae. Zittel.1 
Ambulacra more or less broad, composed of several columns of poriferous plates 
in all genera except Paleechinus and Rhoéchinus, where there are but two. Car- 
boniferous. 
Paleechinus (Scouler), M‘Coy (Fig. 353). Test spheroidal, rigid. dmb 
straight, composed of two vertical rows of low thick plates, extending 
across the half Amb 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. 
IAmb broad, convex, with 
five to eight columns of 
tumid plates, diminishing 
toward the poles. Peri 
proct in apical system, sur- 353. 
rounded hy five large basal, fmoglinns ums wcuy. | Caronions Limestone; Tel 
plates, each perforated by 
one or three canals, and five doubly perforated radial plates. Silurian ; 
England. Carboniferous Limestone ; Great Britain. 
Tthoéchinus, 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. dmb 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 /4mb 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. dmb 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. L4mb 
with four to eleven columns of plates, diminishing in number toward the poles. 
The median plates are hexagonal ; the two rows adjacent to the 4mb 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. fiir Naturgesch. Bd. XXI.), 1855. 
Etheridge, R., On Relationships between the Echinothuridae and Perischoechinidae (Quar. Journ. 
Geol. Soc. vol. XXX.), 1874. 
Keeping, W., On the Discovery of Melonites in Britain (Quar. Journ. Geol. Soc. vol. XXXII.), 1876. 
Duncan, P. M., On the Anatomy of Palzechinus (Ann. and Mag. Nat. Hist. [6] vol. III.), 1889. 
Keyes, C. R., Palaeozoic Echinodermata (Proc. Iowa Acad. Sci. vol. II.), 1894. 
Jackson, R. T., and Jaggar, T. A., Studies of Melonites multiporus (Bull. Geol. Soc. America, 
VII.), 1896. 
Jackson, R. T., Studies of Paleechinoidea (Bull. Geol. Soc. America, VII.), 1896. 

