162 HEMIPEDINA. 



rows of primary tubercles in tlie inter-ambulacra, and another has four rows of primary 

 tubercles in the same region of the test. 



The section of which Hemipedina Marchamensis is the type, appears to belong to the 

 middle and upper division of the Oolites, as the only species exhibiting these hexa- 

 grammous and octogrammous characters, are found in the Calcareous Grit, Coral Rag, 

 and Kimmeridge Clay. 



The test of Hemipedina Marchamensis is large, nearly perfectly circular at the equator, 

 and depressed on the upper surface ; it appears to have had a hemispherical form, but 

 unfortunately the upper portion of the body is fractured, and the outline is traced with 

 difficulty, in consequence of adhering matrix. The ambulacral areas are narrow (fig. 1 a), 

 with two marginal rows of tubercles, closely placed together, along the sutural line, two 

 rows of granules separate the tubercles (fig. 1 U), and transverse branches from these 

 bound the areolas ; the poriferous zones, which closely embrace the bosses, are slightly 

 undulated (fig. 1 (5), from the pores being grouped in threes, the septa are thin and elevated 

 on the surface, and there are five pairs of pores opposite each large plate (fig. 1 b). 



The inter-ambulacral areas are four times as wide as the ambulacral (fig. 1 a) ; they 

 are crowded throughout with numerous rows of nearly equal sized tubercles, which, added 

 to the regular rows in the ambulacra, impart a most remarkable tuberculous character to 

 this urchin ; the inter-ambulacral plates near the equator (fig. 1 h), have three 

 large tubercles placed on the same line, and one or two smaller tubercles on their zonal 

 sides, so that at the equator there are eight rows of tubercles ; the bosses are large and 

 prominent, and the tubercles are small and deeply perforated ; estimating the number of 

 plates in each column at about twenty, each area would contain 20 X 2 = 40 X 4 ^ 160 

 tubercles, which multiplied by five areas, gives 800 tubercles in the inter-ambulacral areas 

 of this species ; the areolas are narrow and superficial, and surrounded by a complete 

 circle of small granules (fig. 1 b), the tubercles of the inter-ambulacra are a little larger 

 than those of the ambulacra. 



The base of the test, which is well preserved, is slightly convex at the sides, and de- 

 pressed at the centre (fig. 1 a) ; the regular distribution of the numerous tubercles gives 

 this region of the shell a highly ornamented appearance, which is admirably represented in 

 (fig. 1 a), the mouth opening is not quite one third the diameter of the body, the peristome 

 is unequally decagonal, the ambulacral lobes being the largest ; the jaws are large and 

 powerful, and project from the mouth with the teeth " in situ." 



The upper surface of the test is unfortunately absent, the specimen having been de- 

 tached from the rock, without due care having been taken to preserve the part to which it 

 adhered, and which evidently contained the other portion of the shell. 



Affinities and differences. — This splendid urchin resembles Hemipedina CoraUijia, 

 from the Coral Rag, in the number of the tubercles developed on the inter-ambulacral plates, 



