BRITISH FOSSILS. 



Decade III. Plate IL 



URASTER GAVEYI. 



[Genus UKASTER. Agassiz. (Sub-kingdom Eadiata. Class Echinodermata. Order 

 Asteriadse. Family Urasterisc.) 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.] 



Diagnosis. U. hrachiis quinis, lanceolatis ; pagind superiori valde 

 spinosd, spinis brevibus, teretibus ; ossiculis ambulacralibus Imearibus^ 

 arcuatis^ carinatisy carina extus sulcata intus fossatd. 



Description.— T]A^ remarkable star-fish is of the shape and dimen- 

 sions of the common living Uraster rubens, to which it presents also 

 a striking resemblance in the details of its structure. Indeed, were 

 this lias fossil to be resuscitated and cast upon our shores now, 

 the British naturalist would see in it only a new but by no means 

 surprising form of Uraster, having close affinities with, and requiring 

 to be distinguished by critical characters from, the commonest of our 

 native Asteriadce. 



The under surface of the specimen is the one exposed, but sufficient 

 of the upper side is exhibited at the sides of the arms to show its 

 structure and armature. The whole of the upper surface of the rays 

 and disk appears to have been covered v/ith .thickly- set, rather short, 

 tapering spines, mingled with smaller and slenderer ones, and traces 

 of pedicellarice. The ambulacra are bordered by closely-set narrow' 

 plates with steep inner sides, and having their upper surfaces convexly 

 curved and indented by about five depressions, indicating the sockets of 

 as many rather short, slender, and rounded marginal spines. The 

 ambulacra themselves are rather wide and flattened ; down the centre 

 of each is a shallow grove marking the line of junction of the ambula- 

 cral ossicula. These are very narrow and linear in shape, slightly bent, 

 with the appearance of a very shallow sigmoid curve. This is caused 

 by the curved keel which runs down each, grooved throughout two- 

 thirds of its length, but depressed and marked with two pit-like 



[ill. ii.] 



