BRITISH FOSSILS. 



Decade I. Plate IV. 



PROTASTER SEDGWICKII. 



[Genus PEOTASTER. Forbes. (Sub-kingdom Radiata. Class Echinodermata. 

 Order Ophiuridse. Family Euryales.) Body circular, covered with squamiform plates ; 

 genital openings in the angles of junction of the arms beneath; arms (simple) formed of 

 alternating ossicula,] 



Species Unica. Protaster Sedgwickii. Forbes, 



The remarkable fossil, of which, through the co-operation of Pro- 

 fessor Sedgwick, we are enabled to give a figure, is unique of its kind, 

 and has a peculiar interest in being the only representative of the tribe 

 Euryales as yet discovered in the fossil state. I refer it to that tribe, 

 and not to the Ophiurce, with which at first sight it might be con- 

 founded, on account of the alternation of the ossicula of the arms, a 

 conformation of which the Astrophyton and other genera of the former 

 group exhibit examples, whilst it is never seen in any genus, as yet 

 discovered, of the latter division. Like many other Silurian fossils, this 

 one affords but imperfect materials for an investigation zoologically of 

 its characters ; such, however, as are to be seen in the specimens 

 hitherto procured, are sufficient to enable us to assert with certainty 

 that it is distinct both generically and specifically from any known star- 

 fish, whether recent or fossil. 



Description. — The specimen figured consists of tolerably preserved 

 impressions of the upper and under side of the disk or body, and of por- 

 tions of the arms. The disk is circular, and shaped like that of an 

 Ophiura. The arms are five in number, very narrow, equidistant, and 

 similar. 



The upper and under surfaces of the disk were covered by small, 

 similar, more or less regular, polygonal or crescentic plates, imbricated 

 in scale-fashion, and having punctated surfaces. Those of the under 

 side of the body are smaller and more regular than those of the upper. 

 The mouth is central, and rather small in proportion to the disk. The 

 buccal apparatus is composed of ten parts or processes, arranged in 

 pairs ; half of each springs from the origin of each arm in a diverging 

 manner, and meets the corresponding half to form a lanceolate tooth- 

 [i. iv.] E 



