FROM THE GREAT OOLITE. 81 



Description. — This beautiful little iircliiu was first discovered in the Etage Bathonien of 

 Langrune, Calvados, the true equivalent of the Great Oolite of English geologists. It was 

 entered in M. Agassiz's ' Catalogus S jstematicus ' * as TIemicidaris minor, from specimens 

 sent to him by M. Michelin. It afterwards found a place in the ' Catalogue raisonne des 

 Echinides ' of Agassiz and Desor, accompanied with this remark : " Se distingue entre 

 tous les Hemicidaris par les tubercules tres espaces, dont il n'y a que deux ou trois dans 

 une rangee. Terrain Jurassique de Erance." — Michelin. Professor M'Coy, in his paper 

 ' On some new Mesozoic Radiata,' f afterwards described this urchin under the name 

 Acrosalenia rarispina, giving the Great Oolite of Minchinhampton for its locality. 



As that gentleman has kindly favoured me with pen-and-ink sketches of the species 

 described as new in his paper, I have no difficulty in deciding on the identity of his 

 specimen. Moreover, I have ascertained the collection from whence it originally came. 

 The error committed by this author in the genus must have arisen from the disc in his 

 urchin having been covered with " adhering siliceous matrix," and from his having 

 overlooked the very remarkable character pointed out by Agassiz, " les tubercules tres 

 espaces." 



I have been fortunate enough, through the kindness of my friend Prof. Deslongchamps^ 

 to receive a typical specimen of the original species from the Great Oolite of Langrune, 

 which I have compared with specimens obtained from the same locality as that 

 whence Professor M'Coy's was collected, and there is not a shadow of a doubt about 

 their perfect identity. This pretty little Hemicidaris is very distinct from all others of 

 the group to which it belongs. The test is nearly hemispherical (fig. 5 a), and the few 

 primary tubercules stand prominently, at great distances apart from the surface of the test. 

 The narrow ambulacral areas are shghtly flexuous above, and have from four to six semi- 

 tubercles at their base ; the sides and upper part of the areas having first four (fig. 5 c), 

 and then two rows of small, imperforate granules upon their surface, about equal in size to 

 the granulation which covers other parts of the test. 



The poriferous zones are depressed, and the pedal pores disposed in pairs through- 

 out (fig. 5 c) ; there are twelve pairs of pores opposite each of the large tubercular plates. 

 The inter-ambulacral areas depart considerably from the typical structure of this portion 

 of the test in the Hemicidarida (fig. 5 a) ; they are three times the width of the 

 ambulacra, and have at their base three large primary tubercles, two on one side and 

 one on the other, with a smaller tubercle above the single large one (fig. 5 «5) ; on the sides 

 and upper part of the areas there are only three primary tubercles, two on one side and 

 one on the other, making only three pairs of primary tubercles in the inter-ambulacral 

 areas, those of the base being closely set together, and those on the sides at great distances 

 apart (fig. ^ a, b) ; the tubercles are large and hemispherical, and only shghtly perforated 

 (fig. 5 c) ; the mammillary eminences which support them are small and ring-like, (fig. 5 a), 



* 'Catalogus Systematicus Ectyporum Echinoderm. Foss. Mus. Neocomensis,' 1840. 

 t 'Annals of Natural History,' 2d series, vol. ii, p. 411. 



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