_ Echinoid Tests, Terminology. 61 
on the observer’s left hand is designated a, that on his right hand b, The plates in each 
column are numbered 1, 2, 3, etc., starting from the adoral end. 
A fully developed Interambulacral has 5 sutural margins. In such a plate of a b 
column the margins, taken in a solar or clock-wise direction, are adradial, adoral, 
orad-interradial, apicad-interradial, andadapical. The same terms may be 
applied to an ambulacral, with the substitution of «-perradial» for «-interradial». In both 
cases the adoral and adapical margins meet in the transverse sutures, and the inter- 
radial or perradial margins in oblique sutures, 
In nearly all the Triassic Echinoidea each interambulacral bears only one primary 
tubercle, which may therefore be more briefly referred to as the main tubercle. 
The smooth space surrounding it, and defined in life by the attachment of the external 
radiolar muscles, is the scrobicule, The surface of an: interambulacrum (as also of a 
single interambulacral) is therefore either intrascrobicular or extrascrobicular, 
In any single plate the term scrobicule is frequently extended to the whole intra-scrobi- 
inter- 
parapet of poral 
boss foramen mamelon platform space bevel 
basal terrace 
scrobicule 
scrobicular circle 
scrocicular ring 
bevel 
inter- inter- ad- ad- ' La 
radial tuber- radial radial i radial 
tract cular tract tract tract 
tract 
extrascrobicular surface poriferous tract 
(pore-field) 
Text-fig. 8. Terminology of Echinoid interambulacrals and ambulacrals. 
cular surface: thus, the radius of a scrobicule being a straight line from the centre of 
the tubercle to the outer edge of the scrobicule, the diameter is twice that radius; and 
a scrobicule is described as large or small according as its diameter is greater or less 
in proportion to the size of the plate. On the other hand, the width of a scrobicule is 
measured from the outer edge of the boss to the outer edge of the scrobicule; and a 
scrobicule is described as broad or narrow according as its width is greater or less 
in proportion to its diameter as above defined. Thus a large scrobicule may be narrow and 
a small scrobicule may be broad. Neither diameter nor width includes the scrobicular ring 
(vide infra). The extrascrobicular surface may be divided into the following tracts: a 
meridionally continuous median interradial tract (the «miliary zone» of Drsor); a 
meridionally continuous adradial tract on each side of the interambulacrum; a disconti- 
nuous meridional series of intertubercular tracts in each column, aborted when the 
scrobicules are contiguous or confluent. (Text-fig. 8.) 
An Ambulacrum may be divided into five continuous meridional tracts: two outer 
