﻿FROM THE UPPER CHALK. 



231 



of E. subrotundus, of which it may be only a variety ; it has, however, fewer tubercles on 

 the inter-ambulacral plates, and the vent is much higher up on the test than in that 

 species. Its globose form presents a remarkable contrast to the conical E. couicus, the 

 elongated E. castanea, and the recurved posterior base of E. abbreviatus. 



Locality and Stratigraphical Position. — This very rare species is found in the Upper 

 White Chalk with flints at Gravesend and in Kent. The test I have figured is contained 

 in the British Museum. 



Genus — Holectypus, Desor, 1847. 



Discoides (pars), Klein, 1/34. 

 Echinites (pars), Leske, 1778. 

 Galerites (pars), Lamarck, 1816. 

 Discoidea (pars), Gray, 1835. 



The Genus Holectypus was established by M. Desor for the reception of those 

 Discoidese 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 Echinoconidse, 

 and are met with chiefly in the Oolitic rocks. They form, according to the views of the 

 late Professor Porbes, " a section or sub-genus of the Galerites, more valuable on 

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

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

 character 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, with six or eight rows of 

 small tubercles, of which the marginal series 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. 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. 



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

 is divided by obtuse notches into ten equal lobes. The organs of mastication consisted 

 of five jaws, which are preserved in situ in one specimen I collected from the Forest 

 Marble of Wilts. 



