258 



Family 6— ECHINOCONID^. 



Galeeites (pars), Besor. 1842. 

 EcHiNONEiDES (pars), Agassiz and Besor. 1847. 



This natural family includes urchins which have the mouth opening central, or 

 sub-central, and the vent excentral. Their test is thin, vi'ith a circular or sub-pentagonal 

 circumference ; the upper surface in general is elevated, and sometimes even conical ; 

 the ambulacral areas are simple and lanceolate ; the poriferous zones extend without 

 interruption from the mouth to the apical disc, and the pores are unigeminal, except near 

 the mouth, where they lie in triple oblique pairs. The inter-ambulacral areas are wide, 

 the tubercles are small and perforated ; they are arranged Avith more or less regularity 

 on the plates, and supported on bosses which have either smooth or crenulated summits. 

 In some genera, they form regular vertical and concentric rows, and the inter-tubercular 

 surface is covered with small granules, which form complete circles round the base of the 

 tubercles. 



The mouth opening is circular, central, or sub-central, and the peristome is more or 

 less divided by notches into ten lobes. The organs of mastication consist of five jaws, 

 which appear to resemble those of the Echinid^. 



The anal opening is always large and excentral ; it has an oblong or pyriform shape, 

 and is either dorsal, marginal, infra-marginal, or basal, sometimes occupying the entire 

 space between the mouth and the border. 



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

 plates ; the right antero-lateral plate is very large, and extends backwards into the centre 

 of the disc ; it supports on its surface a prominent, spongy madreporiform body ; 

 the anterior and posterior pairs of ovarials are perforated, whilst the single posterior plate 

 is imperforate ; and the five small ocular plates are perforated near the margin. 



The spines are small, short, conical appendages, with a smooth head, and having 

 the stem covered Avith longitudinal microscopic lines, 



I include in this extinct family the genera Holectypus, Discoidea, EcJiinoconus, Pyyaster, 

 and Hyhodypus. The Holectypi are found mostly in the Oolitic rocks, the Discoidea 

 and Echinoconi are true Cretaceous forms, the Hyboclypi are Oolitic, and the Pyyasters are 

 common to the Oolites and Chalk, although they had their greatest development during 

 the Oolitic age. The oldest types are the Fygasters and Hyboclypi, which were created 

 about the same time, at the commencement of the deposition of the basement beds 

 of the Inferior Oohte. 



