214 STOMECHINUS. 



The poriferous zones are wide ; the pairs of pores are disposed in oblique ranks of 

 threes, rather widely asunder (fig. 1 b), and between each rank two small tubercles 

 are developed, which separate them from each other throughout the zones (fig. 1 b), 

 so that there is only one trigeminal rank of pores opposite each large inter-ambulacral 

 plate. 



The inter-ambulacral segments are twice the width of the ambulacral ; the centro- 

 suture is very distinctly marked, but there is no median depression, and the division of 

 the test into fifteen lobes, so characteristic of Polycyphus, is not seen in this form ; each 

 of the large plates in general supports four or five tubercles, which are rather irregularly 

 disposed on its surface (fig. 1 b) ; they have narrow, ring- like areolas around their base, 

 encircled by rows of small granules (fig. 1 b), invisible to the naked eye, and only 

 seen with a lens ; the inter-ambulacra have therefore ten rows of tubercles at the equator ; 

 the third tubercle from the poriferous zones represents the principal row, which is con- 

 tinuous from the base to the disc ; but the other lateral rows disappear at different points 

 between the circumference and the vertex. 



The apical disc is unfortunately absent, and the base is adherent to a hard shelly 

 fragment of the Great Oolite limestone, which cannot be removed without, at the same 

 time, detaching the shell. In this remarkable formation, the Oolitic grains are indented 

 into the plates of the test of the Echinodermata, as well as into the shells of the Mollusca ; 

 and unless the specimen separates readily from the matrix, it is almost hopeless to expect 

 to remove it without so much injury to the sculpture as will render its determination doubt- 

 ful, or even impossible. Eor this reason I have been unable to expose the base of this 

 beautiful rare form. 



Affinities and differe7ices. — The nearest affinities of this species is with Folycyplius 

 Normannus, Desor; but it has a much thicker test, the inter-ambulacral tubercles are 

 proportionately smaller and more irregularly disposed, and they have circles of granules 

 around their areolas ; the segments are not divided into lobes by median depressions ; 

 the poriferous zones are wide and only slightly depressed, and the trigeminal ranks of pores 

 are separated by small tubercles : all these diagnostic characters show how distinct 

 Stomechinus microcyplms is from Pohjcyphus Normannus, when a critical comparison is 

 made between these two urchins, which are often found in the same bed. I know of no 

 other form for which our species could be mistaken. 



Locality and Stratiyraphical position. — This beautiful urchin is one of the many 

 exquisite fossils collected by my friend Mr. Lycett, from the Great Oolite of Minchin- 

 hauipton Common. It occurs in the shelly beds of limestone at the large quarry, and 

 is the only specimen found by him, after many years' diligent search, in his favorite 

 localities. 



I have one small specimen of the same urchin, collected from the Great Oolite of 



