148 HEMIPEDINA. 



Locality and Stratigrapldcal position. — This beautiful little Diadem has been found in 

 the Marlstone of Bredon, Alderton, and Dumbleton Hills, Gloucestershire, and in the 

 same rock at Ilminster, Somersetshire. It is everywhere a very rare form. I dedicate 

 this species to my friend Sir WilKam Jardine, Bart,, of Jardirie Hall, Dumfrieshire, 

 author of a magnificent work on the ' Ichnology of Annandale,' and other valuable treatises 

 on different branches of Natural History. 



Hemipedina Etheridgii, Wri(jU. PL IX, fig. 5 a, b, c, d, e,f,g. 



Pedina Etheridgii. Wright, Annals and Magazine of Natural History, 2d series, 



vol. xiii, p. 315, pi. 11, fig. 5 a — c. 

 — — Morris, British Fossils, 2d edition. Additional species. 



Hypodiadema Etheridgii. Desor, Synopsis des Echinides Fossiles, p. 61. 



Test circular, depressed; ambulacral areas with from six to eight small perforate 

 tubercles at their base, and a double row of small granules on their upper surface ; the 

 inter-ambulacral areas with primary tubercles only, the areolas of which are surrounded 

 by regular circles of granules ; pedal pores not numerous, arranged in nearly a single 

 file, with a slight elevation between the two pores of each pair ; apical disc large ; ovarial 

 plates leaf-like ; mouth opening small. 



Dimensions. — Height, one quarter of an inch ; transverse diameter, half an inch. 



Description. — This pretty little urchin has a circular outline in the young state, which 

 in larger specimens inclines towards a pentagonal form ; the base is flattened, and the 

 upper surface is much depressed (fig. 5 c). The ambulacral areas are straight and narrow 

 (fig. 5 d, e,f) ; they have from six to eight small perforated tubercles at their base 

 (fig. 5 e), and a double row of from twelve to fourteen minute, imperforate granules in 

 each row on their upper surface (fig. 5 d), which, in form and size, resemble those cover- 

 ing the other parts of the test (fig. 5 y) ; between the pedal pores of each pair is a small 

 elevation (fig. 5 c/) ; these collectively form a prominent moniliform line, which extends 

 from the margin of the disc to the mouth opening ; the pores are unigeminal, and disposed 

 in nearly a single file throughout (fig. 5 (f). The inter-ambulacral areas are about twice 

 the width of the ambulacral ; the rows of primary tubercles occupying the centre of the 

 plates have seven or eight tubercles in each row ; they are small in size, and rendered promi- 

 nent from being raised upon uncrenulated mammillary eminences, the bases of which 

 are sharply defined, and surrounded by complete circles of small scrobicular granules 

 (fig. 5 (/) ; the regular disposition of these granulations gives an air of decoration to 

 this beautiful little species, and the entire absence of secondary tubercles from the 

 areas renders the ornamentation even more complete (fig. 5 d, e,f). The apical disc 



