464 ADDITIONAL NOTES 



Dimenslovs. — Height, seven tenths of an inch ; transverse and antero-posterior 

 diameters of the test nearly equal, two inches. 



Description. — This urchin is remarkable for the length and width of the anal aperture, 

 and for the great size of its mouth-opening ; it is likewise much depressed and penta- 

 gonal, and covered with very small tubercles, sparsely distributed on the plates (fig. 

 5 a) ; in these respects it presents an assemblage of characters which, taken together, pro- 

 duce a form ver}^ different to any of the many varieties of Pygaster semisulcatus which 

 have hitherto passed through my hands ; for these reasons I have separated it from 

 them under a distinct name. Knowing, however, the vdde variations which many species 

 exhibit in different individuals, and how necessary it is to possess examples of a series 

 of these forms for comparison. I am most reluctant, in the absence of such materials, to 

 multiply specific names. Still, for the sake of clearness, the provisional name macrostonia 

 is proposed for this form. 



Having only seen three or four examples of Pygaster macrostonia, the evidence, to my 

 mind, is not sufficient to write positively on the subject, although all these specimens were 

 remarkable for the great size of the two openings in the test. 



Should a number of specimens of this urchin be hereafter gathered, and carefully 

 compared with each other, it will then be seen whether the characters I have pointed out are 

 persistent in the group, or shade off into forms, which may blend with other varieties 

 of Pygaster semisulcatus. In the mean time it is right to register this urchin under a 

 provisional name, and wait for the future discovery of more specimens for determination. 

 The one proposed indicates its characters. 



Part of the apical disc is preserved in a smaller specimen (fig. 5 ^) ; it consists of 

 four ovarial plates, the right antero-lateral supporting the madreporiform body being the 

 largest, the single ovarial plate is absent in this specimen ; the five small heart-shaped 

 ocular plates are wedged in the interspaces between the ovarials (fig. 5 h), forming a 

 crescent around the sub-compact disc ; the posterior margin of the plates is free (fig. 5 h) ; 

 it does not appear, however, in what manner the anal membrane and plates were connected 

 therewith. 



Locality and Blratigrapldcal position. — This urchin was collected in a bed of sandy 

 Oolite, near Hampen, but whether it belongs to the Inferior Oolite or Cornbrash, I have, 

 at present, no means of determining. 



