REVISION OF PALEOZOIC STELLEBOIDEA. 13 



TERMINOLOGY AS APPLIED TO THE SKELETON OF PALEOZOIC 



ASTEROIDEA. 1 



Abactinal. 



See Body-wall. The integumentary skeleton opposite to that having the ambu- 

 lacral grooves, or that of the upper or dorsal side of the animal. Also known 

 as the aboral surface. See Actinal. 



Aboral. 



See Abactinal. 



Accessory plates. 



This term refers to the abactinal longitudinal columns, or transverse rows, of small 

 plates between the radials and supramarginals (=radial accessory plates or dorso- 

 laterals), or between the inframarginals and supramarginals (=ambital plates or 

 intermarginals), or where there are no radials, to all the ossicles between the two 

 supramarginal columns. There may also be accessory actinal plates which are 

 termed accessory interbrachials, accessory actinals, or inter actinals. The accessory 

 plates form the secondary skeleton and are inserted between the columns of 

 the primary skeleton. It is in the secondary skeleton that much of the 

 specific and generic evolution takes place. See Interbrachial. 



Actinal. 



See Body-wall. The integumentary skeleton with the ambulacral grooves, or 

 that of the under or ventral side of the animal. Also known as the oral surface. 



Adambulacrals. 



The single columns of prominent plates adj oining the ambulacrals . There are never 

 more than two of these columns in a ray, one on each outer side of the depressed 

 ambulacrals. They are generally large and thick and bear the larger, or adam- 

 bulacral, spines . These spines may be inwardly directed and serve as a protection 

 for the tube-feet. The oral armature in Paleozoic species usually consists of five 

 pairs of modified adambulacrals, each pair being the oral projection of two united 

 columns of adjoining rays. They are also called Adambulacralia. Also see Inter- 

 brachial. 



Ambital. 



The abactinal space between the infra- and supramarginal columns, occupied by 

 accessory plates. These are best developed in the axillary disk areas (disk ambi- 

 tals), while those of the rays are referred to as ray ambitals. They are also 

 known as intermarginals. 



Ambulacral. 



Ambulacral grooves (five or more), plates, or ossicles are situated actinally and per- 

 radially along the center of the rays and disk. They are also called ambula- 

 cralia. The ambulacral columns are in pairs, the interlocking movable plates 

 of adjoining columns may be alternate or opposite (always so in recent forms), 

 and the plates of a column may be superposed like the tiles on a roof. The 

 podial openings through which the fleshy tube-feet protrude are situated be- 

 tween the sutures of adjoining plates. In early forms there are but two columns 

 of podial openings in each ray, one on each side of the axial line, but in later 

 forms there may be as many as eight columns. The ambulacral grooves may 

 be tapering or petaloid. 



Anal opening. 



The only Paleozoic form in which an anal opening may exist visually is Hudson- 

 aster. Here it is on the disk between the central plate and the madreporite. 

 The anal pore is nearly always obscured in fossil starfishes, and is absent in some 

 recent species. 



1 Also in the main applicable to the Auluroidea and somewhat to the Ophiuroidea. 



