256 Dr. T. Wright on the Cidaridse of the Oolites. 



Hemicidaris alpina, Agass. PI. XI. fig. 3 a, b. 



Syn. Hemicidaris alpina, Echin. Foss. Suisse, Agass. t.l8. fig. 19-22. 



Test subglobose; ambulacral arese undulated, prominent and 

 convex, covered with small hemispherical granules closely set 

 together ; base of the arese with four mammillated and perfo- 

 rated tubercles -, apical disc large, convex and prominent. 



Height nearly y^^ths of an inch, transverse diameter y%ths of 

 an inch. 



Description. — The test of this beautiful species is subglobose ; 

 the ambulacral arese are slightly undulated and of a medium size ; 

 they are prominent and convex, of an elongated conical form, and 

 are thickly covered with small hemispherical granules without 

 perforations or other sculpture ; the marginal rows are larger and 

 more regular. Between them are from four to six rows of smaller 

 granules closely set together. 



At the base of the arese are four mammillated and perforated 

 tubercles which are limited to this region. The pores are set 

 obliquely in pairs with a smooth elevated granule between each 

 pair, which forms a moniliform sinuous line running between the 

 pores. The interambulacral arese are of moderate breadth, with 

 two rows of primary tubercles, five or six in each column. The 

 mammillary eminences of the two central tubercles are large and 

 prominent. Those towards the anal and oral poles are smaller; 

 they are all crenulated at their summits; the tubercles are 

 deeply perforated, and supported on a short stem, the hemisphe- 

 rical head of the tubercle not exceeding in diameter that of the 

 stem ; the ai'colse around the mammse are slightly channelled 

 and nearly all confluent, those towards the anal pole have a circle 

 of granules encircling the areolse; the interareolar spaces are 

 covered with small smooth granules similar in form and size to 

 those occupying the ambulacral arese. The apical disc is promi- 

 nent, the ovarial plates are large, convex, and much granulated, 

 and the ocular plates are of a proportionate size ; the spines are 

 unknown. 



The mouth-opening is of moderate size, its margin being 

 deeply notched and reflexed as in H. intermedia ; the pores are 

 disposed in simple pairs all the length of the poriferous avenues, 

 but are arranged in double files around the border of the oral 

 aperture in such a manner as to occupy the free space in the 

 ambulacral arese, resulting from the contraction of the interam- 

 bulacral arese in this region. 



Affinities and differences. — Our specimen is smaller in size than 

 the one figured by Agassiz from the Calcaire de Saanen. The 

 ambulacra are more prominent and convex than those of the 

 Swiss specimen ; the rows of marginal granules are not so pro- 



