BRITISH FOSSILS. 



Decade I. Plate I. 



SILURIAN SPECIES OF URASTER. 



[Genus URASTER. Agassiz. (Sub-kingdom Radiata. Class Echinodermata. Order 

 Asteriadse. Family Urasteriaj). Body stellate, five-rayed ; a vent on the dorsal surface ; 

 rays rounded, surface spinous ; ossicula small, compressed, irregular, reticularly combined ; 

 ambulacra bordered by three sets of spines ; suckers quadriserial. The genera Astera- 

 CANTHioN of MuLLER and Troschel, and Asterias (restricted) of J. E. Gray, are 

 synonymous with Uraster.] 



Fig. 1. 

 URASTER RUTHVENI. 



E. Forbes, in " Memoirs of the Geological Survey of Great Britain," 

 vol. ii. part 2, p. 463, 



Diagnosis. U. brachiis quinis teretibus, longissimis, angustis^ subcari- 

 natis ; disco parvo ; pagind superiori reticulata, spinosd, spinis obtusis 

 fasciculatis. Ossiculis ambulacralibus linearibus, longis, geniculatis. 



Description. — Body very small, in the centre of five tapering linear 

 lanceolate, rounded rays, subcarinated on their upper surfaces^ five 

 times as long as the disk is broad. The upper surfaces of both rays 

 and disk are reticulated^ indicating a structure which originally in all 

 probability consisted of spines grouped in tufts. The under surfaces 

 are marked by the impressions of a double series of ambulacral articu- 

 lations, each slightly curved. Both these structures are present in 

 some existing antarctic forms of Uraster. The largest specimen 

 examined measured three inches and a half across, from arm-tip to 

 arm-tip. 



Affinities. — The slender lanceolate arms distinguish this species from 

 any of its palaeozoic allies as yet described. In general form it re- 

 sembles some living species. 



Locality and Geological Position. — This interesting starfish was dis- 

 covered by Mr. John Ruthven, in strata of the Ludlow division of 

 Silurian rocks at Scalthwaiterigg, and also at Highthorns, both near 

 Kendal, in Westmoreland. Professor Sedgwick, in whose collection it 

 is contained, kindly communicated the specimen for description and 

 delineation. 



[1. i.] 



