20 Mr. H. Seeley on Cambridge Palaontology : — 



Diadema intertuberculatum. 



Circular, tumid, flattened and slightly concave below, somewhat 

 flattened above. The ambulacral and interambulacral areas 

 have each two distinct rows of tubercles, of about eleven each ; 

 the tubercles are large and wide apart. Areolse large, circu- 

 lar, and margined by circlets of granules ; in the interambu- 

 lacral area the borders just coalesce in the longitudinal direc- 

 tion. In the ambulacral area the tubercles are rather smaller, 

 above much smaller, and the areolae separated from each other 

 by a granulated space. The rows of tubercles are very close 

 together, so that the areolae touch obliquely, and on the lower 

 part of the shell are separated by a zigzag line of granules. 

 The interambulacral rows are wide apart, and bordered exter- 

 nally on the base with short rows of about six secondary 

 tubercles. Also, on the basal part, between the primary rows, 

 are two rows of secondary tubercles, each formed of about 

 five or six. On the upper part there is generally a row of 

 granules between those margining the areolae. Shell thin. 



Width y| inch, height -J-^ inch ; width of mouth y^ inch. 



This species differs from D. Carteri in the presence of secon- 

 dary tubercles between the interambulacral primary rows, in the 

 more inflated form, smaller mouth, greater space between the 

 tubercles, &c. 



Rare. Cambridge and Ashwell. Coll. University. 



Diadema {"iTetragramma) scriptum. 



Depressed, very slightly pentagonal, inflated. Pores close to- 

 gether and in double series above, wide apart below. Ambu- 

 lacral areas half the width of the interambulacra, furnished 

 with two rows of about twenty or twenty-two tubercles, which 

 are as large as those of the interambulacra, except on the upper 

 part, where they become smaller and closer together : the rows 

 are close together. As in D. divergens, the tubercles are semi- 

 radiate. On the lower part of the shell the areolae are con- 

 fluent, above they scarcely exist. Round the circumference 

 the intermediate space is granulated; the granules partly 

 surround the areolae, and are for the most part in two rows. 

 Interambulacral tubercles in two primary rows of sixteen or 

 seventeen ; each is flanked externally by a row of nine or ten 

 secondary tubercles, of the same size as the primaries. Out- 

 side of each of these are two tertiary rows, of about twice the 

 number. Areolae on the lower part confluent ; above circular, 

 and commonly margined with a circle of granules, but for 

 which the upper part of the space between the primary rows, 

 which is concave, would be naked. Round the circumference. 



