CIDARIS FROM THE WHITE CHALK. 65 



Description. — This beautiful Cidaris, formerly identified by Professor E. Forbes as a 

 variety of C. sceptri/era, and afterwards catalogued by him as a distinct species, under the 

 name C. sulcata, had been previously considered by the Abbe Sorignet, from its spines 

 alone, as a new form, and described in 1850 as C. hirudo in his work on the ' Oursins 

 fossiles du departement de I'Eure,' and this name has the priority. 



The test of the fine specimen belonging to Henry Willett, Esq., F.G.S., and 

 fully illustrated in PI. IX., fig. 1 a, h, c, attains a moderate size, is inflated at the 

 equator, and equally flattened at the poles. The ambulacral areas are narrow, sinuous, 

 and band-like, forming prominent, wefl-marked segments in the test, filled with small, 

 close-set, regularly arranged rows of granules ; there are six rows at the equator, 

 four rows in the upper and lower thirds, and two rows only near the discal and 

 oral apertures. The most prominent granules are in the external rows; they are 

 slightly mammillated and extend throughout the entire area. PI. IX, fig. 3 b, is an equa- 

 torial inter-ambulacral plate, with the ambulacral area attached, magnified three diameters. 

 In the second row the granules are nearly as large, and extend through eight tenths of the 

 area ; the third rows extend through the height of two large plates ; the granules are 

 set very closely together, and regularly arranged in transverse rows. The poriferous zones 

 are narrow, depressed and flexuous, fig. 2 (5 ; the pores are small and round, and the septum 

 supports a small round granule ; the entire series of septal granules forms a moniliform 

 line between the pores, which gives an apparent lateral extension to the width of the area ; 

 and opposite one of the large plates there are twenty-three pairs of pores. 



The inter-ambulacral areas are wide, the plates composing them being deep and broad, 

 five or six in each column ; the areolas are circular and moderately depressed ; they are 

 closely approximated on the under side, wider apart at the equator, and still further apart 

 above. 



PI. IX, figs. 1 and 3, PI. X, figs. 1,3, 3, show the character of t'he areolae ; the margin 

 is slightly elevated (PI. IX, fig. 3 H) and encircled by a row of larger mammillated 

 granules ; the uppermost plate in each alternate column has a rudimentary wart-like 

 tubercle without areola (PI. IX, fig. 1 a, h) ; and in the other column the tubercle is small, 

 but complete, and surrounded by a narrow shallow areola (fig. \ a, b) ; the base has a 

 smooth flat summit, with only rare indications of crenulations ; those in fig. 3 b, are 

 strongly drawn : the tubercle is large and perforated. The miliary zone is wide, depressed 

 along the middle of the area, and along the transverse lines of the sutures : the granula- 

 tions on the surface of the plates arc large in size and uniform in arrangement thereon. 



The apical disc (PI. IX, fig. 3 a) is large, and composed of five ovarial and five 

 ocular plates ; the rhomboidal ovarials are widely perforated, and the cordate ocidars have 

 small marginal orbits in the disc (fig. 3 c) ; the aperture is pentagonal, and there are in- 

 dications of the outer series of small anal plates. 



The mouth-opening (PI. IX, fig. 3, and PI. X, figs. 1 a, and 3 b) is small, and the 

 peristome slightly pentagonal ; in PI. IX, fig. 3, there is a rudiment of a jaw and tooth. 



