ORDER i PAL^ECHINOIDEA PERISCHOECHINOIDA 219 



Order 1. CYSTOCIDAROIDA. Zittel. 



Test exocyclic, globular or ovoid, thin. Ambulacra narrow, straight, with four 

 r, ,iir,il rows of plates, each with a central pair of pores. Interambulacra broad, 

 ?//'/// n n-merous 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. Thorns. (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 Echinoidea and Cystoidea. 



Order 2. BOTHRIOCIDAROIDA. Zittel. 



Test regular, more or less spherical. Interambulacra with one, and ambulacra 

 ////// 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- 



FIG. 350. 



s Palilcni, Schmidt. Ordovician ; Nommis, Esthonia. A, Test of the natural size. B, Apical 

 system, enlarged. (7, 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- 

 Inihh'ral plates with levelled edges, and usually overlapping. Devonian to Permian. 



Lepidocentrus, Miiller (Fig. 351). lAmb with five to eleven columns of plates 

 at the ambitus. Amb very narrow, with low broad plates, and continued 

 beyond the peristomial margin to the true mouth. Tubercles distant ; spines 

 small, subulate. Devonian ; Europe. 



