259 



Genus— YiOLECTY^US, Besor, 1847. 



DiscoiDES (pars), Klein. 1734. 

 EcHiNiTES (pars), Leske. 1778. 

 Galerites (pars), Lamarck. 1816. 

 DiscoiDEA (pars), Gray. 1835. 



The genus Holedypus was established by M. Desor for the reception of those 

 Discoidea which are deprived of ribs, or projecting processes, on the inner wall of 

 the test; the species referred to this group constitute one of the oldest types of the 

 EcHiNOCONiD^, and are met with chiefly in the Oolitic rocks. They form, according to the 

 views of the late Professor Forbes, " a section or sub-genus of the Galerites, more valuable 

 on account of their palaeontological merits, and limited distribution in time, being in the 

 main characteristic of the Oolitic period, than for the zoological importance of the 

 characters of their organization, which are rather transitional than distinctive."* 



The test is thin, circular, or sub-circular, more or less hemispherical, conical or sub- 

 conical, always tumid at the sides, and flat or concave at the base. 



The ambulacral areas are narrow, straight, and lanceolate, v^ith six or eight rows 

 of small tubercles, of which the marginal rows only extend from the base to the apex. 



The poriferous zones are narrow, and the pores are unigeminal throughout. 



The inter-ambulacral areas are three times the width of the ambulacral, the large 

 pentagonal plates support numerous small, perforated tubercles, which are very regularly 

 arranged in vertical and concentric rows (PI. XVIII, flg. Id). They are raised on bosses 

 with crenulated summits, and surrounded by ring-like areolas ; numerous minute granules 

 are scattered over the surface of the plates, and form circles around the tubercles 

 (PI. XVIII, fig. 1 e). 



The mouth opening is circular, and situated in the centre of the base ; the peristome 

 is divided by obtuse notches into ten equal lobes (fig. 1 b). The organs of mastication 

 consisted of five jaws, which are preserved in situ in the specimen figured at 1 y. 



The anal opening is large, inferior, infra- marginal, rarely marginal, sometimes 

 occupying the entire space between the mouth and the border. 



The apical disc is nearly central and vertical, composed of five ovarial and five ocular 

 plates ; the right antero-lateral ovarial is much the largest, and extends into the centre of 

 the disc; it supports a prominent, convex, madreporiform body; the anterior and 

 posterior pairs of ovarials are perforated, whilst the single plate is imperforate ; the five 

 ocular plates are small, triangular bodies, with marginal perforations (fig. 1 i). 



* ' Memoirs of the Geological Survey,' Decade III, pi. 6, Holectypus hemisphcericus. 



34 



