BRITISH FOSSILS. 



Decade III. Plate V. 



HEMICIDARIS PURBECKENSIS. 



[Genus HEMICIDARIS. Agassiz. (Sub-kingdom Radiata. Class Echinodermata. 

 Order Echinidse. Family Cidarites.) Body sub-globose. Interambulacral segments very 

 broad, bearing (few) primary perforated tubercles, placed on crenated mammillary 

 elevations ; ambulacral areas very narrow, furnished with primary tubercles on their 

 lower portions ; pores in single file, except close to the mouth, where they are ranged in 

 threes ; summit crowned with a disk composed of five ovarian and five ocular plates sur- 

 rounding a central anus ; spines of two orders, the primaries long, cylindrical, mostly of 

 considerable dimensions, the secondaries small, compressed. 



Hemicidaris Purbeckensis. Sp. Nov. 



Diagnosis. H. testa suhglohosd, arearum ambulacralium infra tuberculis 

 majoribus remotis ; inter ambulacralium numerosis (8), subdistantibus ; 

 spinis sub-compressis^ Icevibus, ad basin per spatium angustum sfriatis. 



The species of Hemicidaris are difficult to distinguish from each 

 other, unless we have their spines as well as test before us. Fortu- 

 nately, in the present instance, we have both ; although the specimen 

 figured is unique. The body at first glance might be confounded with 

 that of Hemicidaris intermedia, but on closer examination presents dis- 

 tinctive characters, the value of which is confirmed when we see the 

 wholly different features of the spines. 



The body is sub-globose, depressed above, but probably, like its con- 

 geners, liable to variation in this respect. The interambulacral areas 

 are, about the centre of the sides, rather more than three and a-half 

 times the width of the ambulacral spaces in the same region. On each 

 interambulacral plate there is a primary tubercle, erected on the cre- 

 nated summit of an elevated smooth boss, surrounded by a smooth and 

 defined areola. The ambulacral and centro-sutural sides of this areola 

 are marginated by rather scattered rounded granules, but not the upper 

 and lower margins, where the areolae of contiguous plates may be said 

 to be in contact. The areola is wide in comparison with the boss. 

 There are about eight primary tubercles in each row, gradually increas- 

 ing in size as they approach the centre of the sides of the body. The 

 ambulacral spaces bear in their upper portion, and for two-thirds of 



[ill. V,] B 



