160 HEMIPEDINA. 



at the equator, diminishing above to six, four, and two rows ; tubercles of both areas very 

 small and nearly of the same size; sides rounded, upper and under surfaces much 

 depressed. 



Dimensions. — Height, eleven twentieths of an inch ; transverse diameter, one inch and 

 seven tenths. 



Description. — The only specimen I know of this species is the one now figured ; 

 unfortunately the surface of the test has been rubbed smooth in some places, and distorted 

 and broken in others, so that only an imperfect description of this interesting form can be 

 given. 



The ambulacral areas are wide and straight, and retain a very uniform width through- 

 out ; at the equator they have four rows of small tubercles (fig. 4 c), the two marginal 

 rows are the most regular, as regards size and arrangement, there being about twenty-five 

 tubercles in each row, the two inner rows commence below the equator and extend two 

 thirds of the distance between the equator and the apical disc ; they are smaller in size and 

 not so regularly arranged as the outer rows ; each row contains from ten to twelve 

 tubercles ; between these four rows of tubercles small granules are very regularly inter- 

 spersed. The poriferous zones are perfectly straight and very narrow, and the pores are 

 small and unigeminal throughout; there are five pairs of pores opposite each of the 

 inter-ambulacral plates. The inter-ambulacral areas are about twice and a half as wide as 

 the ambulacral ; each plate at the equator supports four tubercles (fig. 4 c), which are so 

 disposed on the consecutive plates of the columns that they form a series of oblique 

 rows (fig. 4 b) ; there are sixteen plates in each column, and in the upper part of the areas 

 the number of tubercles gradually diminishes to three, four, and two on each (fig. 4 a). 

 The tubercles of both areas are very small, and nearly of the same size ; their bosses are 

 flat, and the narrow areolas are surrounded by a circle of small scorbicular granules, 

 which imparts an ornamental appearance to the test. 



The space for the apical disc is large, but as the margin of the aperture is fractured 

 no certain estimate can be made of the probable size of the disc. 



The base is concave, and the mouth opening so much concealed by adhering 

 matrix that its form cannot be accurately made out ; enough, however, is exposed, to 

 show that the peristome is decagonal and formed of unequal sized lobes. 



Affinities and Differences. — This urchin belongs to the type of Hemipedinas with 

 several rows of equal sized tubercles at the equator ; in the number and smallness of its 

 tubercles it is so unlike all its present known congeners that it cannot be mistaken for 

 either of them. In the width of its ambulacra, and in the mode of distribution of the 

 tubercles in oblique rows, it resembles the Dygasters. 



