NO, 3 CASSIDULOID ECHINOIDS — KIER I53 



Genera. — ArcJiiacia, Gentilia. 



Range. — Cretaceous ( Urgonian-Cenomanian ) . 



Comparison zvith other families. — The Archiacidae are distin- 

 guished by the absence or great reduction of a petal in ambulacrum 

 III, and the doubling of pores in ambulacrum III. 



Genus ARCHIACIA L. Agassiz 



Archiacia L. Agassiz, in Agassiz and Desor, 1847. Ann. Sci. Nat., ser. 3, vol. 7, 

 p. 159. Type species, Archiacia sandaliua Agassiz, by subsequent designation, 

 d'Orbigny (1856, p. 284). 



GENERIC DESCRIPTION 



Medium size, elongate, adapically high, in some species very in- 

 flated anteriorly, oral surface flat or sunken around peristome ; apical 

 system very eccentric anteriorly, tetrabasal, madreporite extending 

 posterior to posterior genital plates; petals short or long, anterior 

 petals (II and IV) curving posteriorly, shorter than posterior petals, 

 pores conjugate, in some species outer pore very elongated obliquely; 

 ambulacrum III not petaloid, often in a groove, usually with double 

 series of pore pairs in each half -ambulacrum ; periproct inframar- 

 ginal, longitudinal ; peristome central or anterior, longitudinal or 

 transverse ; bourrelets present, phyllodes broad, double pored, with 

 approximately six pore pairs in each outer series and three or four 

 in each inner. 



Comparison zcith other genera. — Archiacia is most similar to 

 Gentilia, both genera having inflated tests, nondeveloped or short 

 petals III with doubling of the pores in the ambulacral plates beyond 

 the petal. Archiacia differs in having double pores in its ambulacral 

 plates beyond the petals and is probably ancestral to Gentilia. 



Remarks. — The apical system has not been figured in most of the 

 species of this genus. Mortensen (1948, p. 319) suggested, after 

 noting Gauthier's (1889a, pi. 2, fig. 7) inaccurate figure of the apical 

 system in Archiacia palmata Gauthier, that the apical system in 

 Archiacia was typically monobasal. I have studied both the holotype 

 of A. pahnata in the d'Orbigny Collection in the Museum National 

 d'Histoire Naturelle and three specimens in the Lambert Collection 

 at the Sorbonne, Paris. The apical system (text fig. 125) is definitely 

 tetrabasal with genital 2 extending posteriorly. The genital plates 

 are not arranged in a cruciform fashion as shown in Gauthier's 

 figure. I also saw the apical system in Archiacia santonensis d'Orbigny 

 on a specimen in the d'Orbigny Collection, and it is very similar, 



