270 ECHINODERMATA— ECHINOZOA phylum iv 



be represented sparsely throughout the Paleozoic era. They multiply enor- 

 mously in the Mesozoic, and certain families reach their climacteric in that 

 period ; other families attain their maximum in the Recent. As a rule, the 

 species have a very limited vertical ränge, and hence serve admirably as index 

 fossils. The test is often perfectly preserved, but even small fragments are 

 capable of accurate determination, owing to the regulär radial repetition 

 of parts. 



The Classification here followed is based on that given in Jackson's 

 Phylogeny of the Echini; no subclasses are recognised, but the group is 

 divided into seven Orders. The generic descriptions of the Cidaroida and 

 Centrechinoida and the generic and family descriptions of the Exocycloida are 

 essentially those as given in the earlier edition of this work, or in Duncan's 

 Revision of the Genera and Great Groups of the Echinoidea. 



Order 1. BOTHRIOCIDAROIDA Duncan. 



Test regulär, more or less spherical. Interamhidacra with one, and ambulacra 

 with two vertical columns of plates, which do not imbricate. Feriproä within the 

 apical System. 



Family 1. Bothriocidaridae Klem. 



JFith characters of the order. 



The solitary known genus is Bothriocidaris Eichwald (Figs. 366, a; 367, 

 a; 376; 377), from the Ordovician of Esthonia. The test is small, and the 



Fig. 376. 



Bothriocidaris pahleni Schmidt. Ordovician; Nömmis, Esthonia. ^, Test of the natural size. J5, Apical 

 System, enlarged. C, Peristome, enlarged (after F. Schmidt). 



apical System consists of five large ocular and five small genital plates ; 

 periproct plated, peristome with ambulacral plates only. 



Of this important genus, the oldest of known Echini, there are three species, B. archaica, 

 pahleni and globulus. The ambulacra have two columns of high hexagonal plates in each 

 area with pore-pairs superposed in a central peripodium. Interambulacra with one column 

 of plates only in each area. Ocular plates relatively large and meeting in a continuous ring 

 {£. archaica), or partially or wholly separated by the small genitals. Genitals in £. archaica 

 dorsal to the oculars (Fig. 377, B) or in other species partially or wholly separating them and 

 reaching the interambulacra, Bothriocidaris is structurally important because in its high 

 ambulacral plates, with pores superposed, its single column of interambulacral plates, its 

 simple peristome and its large oculars, it presents features like the young of later Echini. All 

 other Echini start with a single interambulacral plate ventrally, representing a single column, 

 and later add one or more columns. Those types with only two columns of plates in an inter- 

 ambulacral area show no evidence of beiug derived from types with many columns, and are 

 therefore considered as next related to the Bothriocidaroida. 



