274 PYGASTER. 



tubercular surface of the plates is covered with numerous small miliary granules, which 

 form circles around the areolas. The basal tubercles are larger than those on the upper 

 surface, their areolas are more excavated, and they have a quadrate or hexagonal figure, 

 which makes a striking contrast to their circular form on the upper surface. 



The apical disc is large, composed of five ovarial and five ocular plates ; the right 

 antero-lateral ovarial is the largest, it extends into the centre of the disc, and supports a 

 large madreporiform body on its surface. The five ocular plates are small, and wedged 

 between the angles left by the ovarials. 



The anal opening is situated on the upper surface of the single inter-ambulacrum ; 

 it is a very large, oblong, or pyriform aperture in the test, and in the living urchin appears 

 to have been closed by a tegumentary membrane ; in all the species at present known, 

 P. pileus excepted, the discal and anal openings are continuous ; but in that species 

 a portion of the test separates the apical disc from the vent. 



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

 the peristome is divided by deep notches into ten equal-sized lobes. Remains of jaws have 

 been found in some species ; and imprints of ten carinse, radiating from the centre to the 

 periphery, are seen in the moulds of others. The Piasters, therefore, possessed mastica- 

 ting organs probably allied in structure to those I have already described in Holectypics 

 depressus, Leske. 



The spines are small, short appendages, which resemble the same parts in Echinus, 

 and, like them, their surface is covered with well-marked longitudinal lines. 



The Piasters form a type of structure which nearly approaches the Hemipedinas, 

 of the family Diademad^e ; like them, the tubercles are perforated, their bosses have 

 smooth, uncrenulated summits, and they are arranged in regular vertical and horizontal 

 rows. The poriferous zones are narrow and unigeminal ; the mouth opening is circular 

 and decagonal ; and they possess organs of mastication. 



Unlike the Diademad^;, however, the Pygasters are true exocyclous urchins, and have 

 a large vent, placed without the circle of the apical disc. They differ from Holectypus in 

 having the tubercles larger, and the vent at the upper surface ; and from Hyboclypus in the 

 absence of the dorsal sulcus in which the vent is situated in that genus. They are 

 distinguished from Clypcus, Echinobrissus, Catopyyus, Pygauius, and other EcHiNOBRissm.E, 

 in having perforated tubercles arranged in rows ; narrow, complete poriferous zones ; and 

 the mouth armed with jaws. They form, in fact, a well-defined group of urchins, separated 

 by prominent organic characters from all the others. 



The Pygasters first appeared in the lowest beds of the Inferior Oolite ; and their different 

 species are discovered in the Cornbrash, Calcareous Grit, Coral Rag, and Kimmeridge 

 Clay. They have been found likewise in the Gault. 



