H. L. Hawkins — Dental Apparatus of Discoidea. 149 



by the Rev. A. Dixon from the Lower Clialk of Sewell, in Cambridge- 

 shire, is what would be usually described as a young form, the 

 cylindrical character of the sides of the test of adult forms being 

 scarcely perceptible. The dimensions of the test are : Breadth of 

 base, 38'75 mm. ; height of test, 22-25 ram. At one point the test 

 was broken away, and by careful development sufficient was removed 

 to expose the five pairs of hemi-pyramids with their teeth, all lying 

 in the left anterior interradius (Fig. 1). 



The maxillce are small and lanceolate in form, with their adoral 

 ends strongly curved inwards. The inter-pyramidal joint-face (Fig. 3) 

 is broad, being 6 '6 mm. long and 3 "2 mm. wide at its greatest breadth, 

 which extends from about 2 mm. above the oral end, almost to the 

 aboral end of the face. Its aboral margin slopes very slightly out- 

 wards and downwards. The transverse ridges are almost parallel to 

 the aboral margin ; they are straight and very fine, being about seven 

 to each millimetre. Theybss« arcualis is high, and about "TS of the 

 breadth of the aboral margin of the inter-pyramidal face. The outer 

 surface (Fig. 2) is slightly narrower, and strongly convex adorally. 

 Its length is 8-1 mm. and its greatest width 3-1 mm. The external 

 margin is sharp and elevated into a flange. The admedian outer 

 plane is very narrow aborally, but becomes widened into an inward 

 sloping plane, TO mm. wide adorally. It crosses the marginal 

 flange, and is continued as a strongly incurved process for about 

 •75 mm. beyond it. The fovea magna externa is narrow and deep 

 below, with a distinct facet on the outer side of the admedian plane, 

 but it rapidly becomes shallower upwards, and, save for a slight 

 depression aborally, it forms a broad, somewhat concave surface. The 

 symphysial f ace is narrow ("9 mm. wide near the upper end), becoming 

 somewhat broader at the incurved adoral end. The length of the 

 symphysis is 5-4 mm. (chord measurement), and the full height of the 

 maxilla is 8-1 mm.; the ratio of symphysis to maxilla is therefore '67. 

 The internal face has a deep concavity partly divided by a carina at 

 the aboral end. The dental slide is concave and rather deep. It 

 extends to within -6 mm. of the angle of the supra-alveolar process. 

 The adoral region of the internal face is smooth. 



The epiphysis (Fig. 6) is very minute and slender. Its horizontal 

 length is 2-7 mm., its height at the outer end 1-5 mm. It is reduced 

 to a narrow bar towards the inner end. The inter-pyramidal surface 

 is horizontallj^ ribbed, with a rounded prominence at about 1 mm. 

 distance from the inner end. The surface of articulation with the 

 maxilla is almost smooth. 



No recognizable trace of a rotula or of a eompass can be determined, 

 but the rounded prominence on the epiphysis indicates the existence 

 of a rotula, as this articulation is the chief one between these two 

 pieces in most Regular Echinoidea. 



The teeth are narrow and but slightly curved. There is a strong 

 keel on the concave side, and a less conspicuous rounded ridge along 

 the convex side, as in Echinus. The cutting edge of the tooth seems 

 not to have been very sharp, but to terminate in a blunt chisel edge 

 rather than in a point. The tooth is, however, a far more effective 

 cutting instrument than the short stumps of the Clypeastroid jaws. 



