270 MICRASTER 



The mouth-opening is found very near the anterior border, figs. 1 b, c, and e. It is 

 largely and transversely oblong, and the lower lip is thick and much prolonged. 



Affinities and Differences. — This Urchin resembles some forms of Micraster cor- 

 anguinum, and has long been considered by many a variety of that common Chalk species. 

 An attentive study of the anatomy of their tests will soon disclose their affinities and 

 differences, which have already been pointed out in the introductory part of the descrip- 

 tion, and to which I refer the reader. 



Locality and Stratigraphical Position. — The most typical examples of this species are 

 found in the Upper Chalk at Harford Bridge, near Norwich, and from the Upper Chalk 

 of Sussex, where it is very rare. The large specimen figured in ' Dixon's Geology of 

 Sussex ' is a broad and abnormal form of the species, and is the type of my late 

 colleague's description in the text of that work. 



Since this article was in type my friend Mons. de Loriol has called my attention to 

 and given me a copy of a memoir by Dr. Clemens Schlatter (' Verh. d. Nat. Ver. 

 Jahrg.,' xxvi, Folge 3, Bd. vi), " Fossilen Echinodermen des nordlichen Deutschlands," 

 in which the author has published good figures and an accurate description of Epiaster 

 gibbus. 



Genus — Micraster, Agassiz, 1836. 



Spatangus, pars, Auctorum. 

 Micraster, pars, Agassiz. 

 Micraster, oVOrbigny. 

 Micraster, De Loriol. 



Body oval, oblong, more or less inflated ; test cordiform, thin, and fragile. 



Ambulacral pairs petaloidal, closed at their extremities, the anterior exceeding in 

 length the posterior pair. 



Poriferous zones equal in each ambulacra. 



Holes oval or oblong, conjugated by transverse depressions. 



Anterior or single ambulacrum lodged in a wide, shallow, anteal sulcus, which 

 deeply indents the border of the test. 



Pores small and round, and set obliquely in uniform pairs more or less widely apart, 

 the distance of one pair from another increasing from the disc to the mouth. 



Apical disc small, central, solid, composed of four perforated ovarial and five perfo- 

 rated ocular plates. 



Madreporiform body small, located in the middle of the disc. 



Mouth-opening transverse near the border ; peristome bilabiate, with a prominent 

 projecting under lip. Base flat or slightly convex, covered with large, regularly arranged, 

 well-developed tubercles. 



