﻿Geology 



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Genus— PYGURUS, d'Orbigny, 1855. 



Echinanthites, Leske, 1778. 

 Clypeaster (pars), Lamarck, 1801. 

 Echinolampas (pars), Agassiz, 1836. 

 Pygurus (pars), Agassiz 1840. 

 Pygurus, d'Orbigny, 1855. 



Pygurus, Desor, 1858. 



The genus Pygurus, as now limited, is composed of large, discoidal, or clypeiform 

 urchins, in which the test in general is more or less enlarged at the sides, and rostrated 

 posteriorly ; its upper surface is usually depressed, and rarely elevated. 



The ambulacral areas and poriferous zones in the upper surface form petaloidal expan- 

 sions, which have an elegant form, being in general contracted at the border, enlarged 

 in the middle, and attenuated at the apex. The anterior single area is narrower than the 

 antero- and postero-lateral areas ; the summit is in general central, or slightly excentral, 

 the inclination being always forwards. 



The base is concave and much undulated, the wide basal inter-ambulacra swell into 

 prominent cushions, and the narrow ambulacra form narrow valleys between them. 



The mouth-opening is pentagonal, and always excentral ; the peristome is surrounded by 

 five prominent lobes, with which five expanded ambulacral petals alternate ; in the pori- 

 ferous zones near the mouth the pores are closely crowded in triple oblique ranks ; 

 these perforated petals form an oral rosette or a penta-phylloid floscule (PI. XXXVII, 

 fig- 1 h, e). 



The vent is infra-marginal ; it is in general oval, and surrounded by a distinct area, 

 which occupies the rostrated portion of the single inter-ambulacrum ; the long diameter 

 of the opening in general corresponds with the longitudinal axis of the test, although it 

 is sometimes transverse (PI. XXX, fig. 2 b). 



The apical disc is very small, and occupies the summit ; it is composed of two 

 pairs of narrow, perforated, and a single rudimentary imperforate, ovarial plate ; five 

 minute ocular plates, with central eyeholes, are interposed between the ovarials (PI. XXXV, 

 fig. 3). The small madreporiform body is attached to the surface of the right anterior 

 ovarial, and forms thereon a spongy eminence, which extends over the other discal elements 

 (fig.3y). 



The tubercles are very small on the upper surface, but larger at the base ; they are 

 surrounded by sunken areolas, have their summits perforated, and the inter-tubercular 

 space covered with close-set miliary granules (PI. XXXV, fig. 2 e, g). 



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