FROM THE INFERIOR OOLITE. 151 



Hemipkdina perforata, Wright. PI. X, fig. 2 a, b, c, d, c,f,(). 



GoNioPYGUS PERFORATUS. Wright, Annals and Magazine of Natural History, 2d series, 



vol. viii, p. 267, pi. 13, fig. 5 a, b. 



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



vol. xvi, p. 98. 



Test small, circular, depressed ; ambulacral areas with two rows of small tubercles, 

 which extend from the peristome to the disc ; inter-ambulacral areas with two rows of 

 tubercles, seven to eight in each row, and three or four secondary-tubercles near the base; 

 mouth opening large ; peristome decagonal, divided into deep, nearly equal-sized lobes ; 

 apical disc large, plates much developed. 



Dimensions. — Height, one quarter of an inch ; transverse diameter, three quarters of 

 an inch. 



Description. — This little Diadem was first figured and described as Goniopyym 

 perforatus, at a time when I was ignorant of the true organic characters of the group to 

 which it belongs, " I have placed this urchin," I observed, ^' provisionally in the genus 

 Goniopygus, as it comes nearer to the character of that form than any other. The absence of 

 crenulations on the mammae, the nearly uniform size of the tubercles, the distinctness with 

 which they stand out from the test, and a fragment of the angular apical disc in situ, seem 

 to justify the supposition of its being a Goniopygus ; but the perforation of the tubercles 

 makes the exception, and suggests the query whether the absence of perforations is a 

 generic or only a sectional character."* At the time this passage was written I had only 

 found three imperfect specimens of this species, and the search after more perfect urchins 

 of the same natural group led to the discovery of several new congeneric forms, and 

 finally to the establishment of the genus Hemipedina for their reception. 



The ambulacral areas are narrow, and carry two rows of small tubercles, which, from 

 the peristome to the equator, are regularly developed (fig. 2 c) ; but above the equator they 

 rapidly diminish in size, and dwindle into small granules in the upper part of the area 

 (fig. 2./, 3, y). 



The poriferous zones are narrow and straight, the septa moderately thick, and at 

 the base of the area the pores fall into triple, oblique pairs (fig. 2/), but in the upper part 

 they are strictly unigeminal. 



The inter-ambulacral areas are twice and a half the width of the ambulacral, and 

 furnished with two rows of tubercles, from seven to eight in each row (fig. '2, b, c) ; the 

 bosses are prominent, and surrounded by a narrow areola (fig. 2 g, the letter omitted) ; 



* 'Annals and Magazine of Natural History,' 2d series, vol. viii, p. 267. 



