GENERA AJ^D GROTJPS OF THE ECHINOIDEA. 135 



V. 



Order III. The Holectypoicla, its Sections and Genera. Order IV. The 

 Clypeastroida, remarks. Family Fibidariidaj and Genei-a. Family Clypeas- 

 tridte and Genera. Family Lagauidse and Genera. Family Scutellid£e, Genera 

 and Subgenera. Subfamily Araclmin£e, and Genus. 



Order III. HOLECTYPOIDA (p. 25). 



Syn. GaleritidcB and EcTiinoconidcB, auct. (pars). 



This is the first Order of the Exocyclica, and it is a difficult 

 one to classify, for there is much diversity of opinion regarding 

 the nature of the perignathic girdles and jaws of some genera (see 

 p. 22). Some genera have all the other characters of the Order, 

 but research has not decided finally upon the presence or absence of 

 jaws and teeth. In other genera the jjerignathic girdle is weak, 

 and there are teeth ; and in two genera the perignathic girdle 

 forms a strong collar and the interradial portions or " ridges " 

 are wide and bent upwards and outwards from the peristome 

 internally. 



The Order may be divided into two sections, and in both the 

 apical system is compact, but the number of the perforate basal 

 plates difi'ers ; the madreporite is in the right anterior basal 

 plate, but it may extend back and separate the postero-lateral 

 basal plates. The ambulacra are straight, flush, and apetaloid. 

 The peristome is decagonal or circular, and has branchial incisions 

 often well developed, or the reverse ; the peristomial margin is 

 composed of two plates in each ambulacrum, and double plates 

 in each interradium, but the odd area may have a single plate. 

 The periproct is variable in its position in the posterior inter- 

 radium. Tentacles are restricted to ambulacral plates. 



Section I. — The perignathic processes of the ambulacra present- 

 Genus Uolectypus. 

 Pileus, 

 Pygaster, 

 Pygastrides. 



Section II. — Perignathic girdle with well-developed interradial 

 ridges ; ambulacral " processes " rudimentary or absent. 



Genus Discoidea. 



Subgenus Echinites. 

 Genus Conoclyp?us. 



Genera incertae sedis : Galeropygus, Pachydypeus. 



