260 PALAEONTOLOGY 



grooves from right and left alternately. Between the 

 margins of the lancet-plates and the radials are side-plates, 

 which in exceptionally preserved specimens bear each a 

 pinnule. From their position in relation to other 

 structures it is obvious that a lancet-plate with its side- 

 plates (sometimes termed an ambulacrum') answers to the 

 arm of a crinoid an arm which is sessile upon and forms 

 part of the calyx. The side-plates bear pores, which 

 open into internal gill-like organs, the hydrospires. The 

 mouth is in the centre of the upper surface : around 

 it are five openings, interradial in position, divided into 

 two internally, the spiracles, which also communicate 

 with the hydrospires. Each spiracle is roofed by the 

 halves of two lancet-plates and their side-plates. The 

 anus is probably confluent with one of the spiracles. 



In another common American species, P. godoni, the 

 base is flattened at the level of the lower end of the 

 lancet-plates. In Orbitremites, found in the Carboniferous 

 Limestone of England, but not common, the calyx is 

 ellipsoidal, and the ambulacra very narrow and extending 

 very far down towards the base. 



The Eleutherozoa are always freely-moving benthic 

 animals, sometimes carnivorous, sometimes mud-eaters, 

 but never microphagous as are all Pelmatozoa (even the 

 few nectic forms) 



Hemicidaris intermedia (Fig. 75) is a sea-urchin of 

 which many very perfect specimens have been found in 

 the Coral Rag of Calne in Wiltshire. The general 

 characters and symmetry of the skeleton suggest at once 



