ORDER I PALEZECHINOIDEA—PERISCHOECHINOIDA 219 
Order 1. CYSTOCIDAROIDA. Zittel. 
Test exocyclic, globular or ovoid, thin. Ambulacra narrow, straight, with four 
vertical rows of plates, each with a central pair of pores. Interambulacra broad, 
with numerous vertical rows of irregular, scale-like plates, each with a small primary 
tubercle surrounded by a scrobicule. Periproct in the posterior interambulacrum 
above the ambitus. Peristome small, with highly-developed jaws. 
Echinocystites, Wyv. Thoms. (Cystocidaris, Zitt.) This aberrant genus, with 
the characters of the order, occurs in the Silurian of Scotland, and was regarded 
by its founder as transitional between the Hehinoidea and Cystoulea. 
Order 2. BOTHRIOCIDAROIDA. Zittel. 
- Test regular, more or less spherical. Interambulacra with one, and ambulacra 
with two vertical rows of plates, which do not imbricate. Periproct in the apical 
system. 
The solitary genus is Bothriocidaris, Eichwald (Fig. 350), from the Ordo- 
C 

Fie. 350. 
Bothriocidaris Pahleni, Schmidt. Ordovician ; Némmis, Esthonia. 4, Test of the natural size. B, Apical 
system, enlarged. C, Peristome, enlarged (after F. Schmidt). 
vician of Esthonia. The test is small, and the apical system consists of five 
large radial plates limiting the periproct, each with two pores ; and five small 
imperforate basal plates. 
Order 3. PERISCHOECHINOIDA. M‘Coy. 
Test regular, globular or ovoid, with more than two vertical rows of interam- 
bulacral plates ; and with two or many columns of ambulacral plates, each with a 
pair of pores. Plates thick and rigid, or thinner and more or less overlapping. 
Family 1. Archaeocidaridae. M‘Coy. 
Ambulacra narrow, each with only two columns of poriferous plates. Interam- 
bulacral plates with bevelled edges, and usually overlapping. Devonian to Permian. 
Lepidocentrus, Miiller (Fig. 351). Amb with five to eleven columns of plates 
at the ambitus. -4mb very narrow, with low broad plates, and continued 
beyond the peristomial margin to the true mouth. Tubercles distant ; spines 
small, subulate. Devonian ; Europe. 
