GLOSSARY. 131 



Genus — Pycinaster. 



Spines are very feebly developed in this genus. The spine-pits when present 

 are very shallow, and often they are absent altogether. Spine-pits are often 

 visible on the actinal plates after they have disappeared from the abactinal series. 



P. crassus possesses rugosities on the distal marginalia. 



Genus — Arthraste b. 



A. cristatus possesses both spines and rugosities; A. Dixoni is rugose without 

 spines. 



Speculation as to phylogeny in these latter three genera, in view of the state 

 of our knowledge, would be valueless. 



GLOSSARY. 



The following glossary and diagram (Text-fig. 34) is added to aid the geologist 

 who has but little acquaintance with modern zoological terms. 



Abactinal. — Applied to the surface Avhich is uppermost when the starfish walks 

 on its tube feet; the term " dorsal" is used by some authors in the same sense. 



Actinal. — Applied to the surface which is undermost when the starfish walks. 

 On this surface are situated the mouth and the ambulacral grooves. The term is 

 used synonymously with " ventral " by some authors. 



Adambulacralia or Adambulacral Plates. — The ossicles which are adjacent to the 

 ambulacral ossicles. In the order " Phanerozonia," to which the great majority of 

 Chalk Asteroids belong, these ossicles are visible on the actinal surface, bordering 

 the ambulacral groove and hiding the ambulacral ossicles. Adambulacralia may 

 be recognised by their prominent armature of spines. 



Adradialia. — Ossicles situated on either side of the radialia (q. v.). 



Ambulacral. — The ambulacral groove is the groove stretching from the mouth 

 to the extremities of the arm. It is formed by the ambulacral ossicles, which meet 

 in the middle so as to form an arch. The tube feet project through the arch and 

 into the groove. 



