15.2 HEMIPEDINA. 



the spinigerous tubercles are small, and not deeply perforated ; they stand out, however, 

 in a well-defined manner, from the surface of the test, and are very uniform in size 

 throughout ; a complete circle of granules surrounds each areola (fig. 2 g), and one row of 

 granules separates the areolas from the zones ; the miliary zone is wide (fig. 2 h), and 

 filled with four rows of round granules, which are very uniform in size, and closely crowded 

 together (fig. 2 g), thus imparting a granular aspect to the surface ; the upper part of the 

 centro-sutural line is naked for the space of the two uppermost plates (fig. 2 (5). 



The apical disc is large (fig. 2 b) ; the five genital plates have a heptagonal form, and 

 are nearly all of the same size ; they are perforated at some distance from the apex, and in 

 the centre of each plate is a cluster of small granules (fig. 2 e) ; the ocular plates are penta- 

 gonal, and large in proportion to the size of the urchin ; they have granules on their 

 surface, and the eye-hole is perforated at some distance from the margin ; the anal opening 

 is round and central, and the madreporiform body is very small (fig. 2 e). 



The base is concave, and the mouth opening, which is situated in a depression, is one 

 half the diameter of the test ; the peristome is decagonal, and divided into nearly equal- 

 sized lobes. 



Affinities and differences. — This species is nearly allied to H. Jardinii and H. Etheridgii, 

 out is distinguished from both by the structure of the ambulacra, in which the marginal 

 tubercles are large and well developed at the base of the area (fig. 2/), but suddenly 

 diminished in size, or rather reduced to granules, in the upper part of the same (fig. 2 g) ; 

 whereas in these other two allied forms they are very regularly developed throughout the 

 area (PI. IX, fig. 4 d, and fig. 5 d, which compare with PL X, fig. 2 h, g). 



Locality and Stratigraphical position. — I collected this species from that remarkable 

 rock, the Pea Grit, at Crickley Hill. The species must have been tolerably abundant, 

 although very few specimens are well preserved. Out of a considerable number I have 

 only obtained two or three in which the structure of the test can be satisfactorily made out. 



Hemipedina tetragramma, Wright. PI. X, fig. 3 a, b, c, d. 



Hemipedina tetrageamma. Wright, Annals and Magazine of Natural History, 2d series, 



vol. xvi, p. 98. 



Test circiilar and depressed ; ambulacral areas narrow, with two rows of small, nearly 

 equal-sized tubercles on the margin of the area, extending from the peristome to the disc ; 

 inter-ambulacral areas with two rows of primary tubercles, about fourteen in each row, 

 and two rows of secondary tubercles, ten in each row, extending from the peristome nearly 



