Echinoid Tests. Terminology. 
65 
it imbrication ; it is due to the arcuate transverse section of the ambulacrum, and their own 
explanation of the slightly inclined edges of the plates in general is equally applicable to 
this particular case. The direction of imbrication has often been stated too vaguely, and 
often, as later observations have proved, quite incorrectly; hence the preceding elementary 
exposition may not be out of place. (Text-fig. 7.) 
Methods of measurement. — All measurements are in millimetres, and 
have been taken with sliding callipers provided with a vernier scale reading to 
tenths of a millimetre. For the sake of comparison between the diagnoses it is 
preferable to express in the form of ratios such measurements as are given therein, 
taking some one measurement as a constant. Unfortunately the fragmentary nature 
of the specimens has not left any single measurement which can conveniently be used 
in this way. Only in a few cases have relative measurements been calculated from 
a constant chosen for each set of cases. 
The h e i g h t of a complete test is the distance measured vertically from the 
vertex to the base-plane, that is the flat horizontal surface on which the denuded 
test assumes stable equilibrium in its natural posture. Owing to the almost universal 
absence of the apical System from even the best specimens of Triassic Echinoids, 
one is reduced to giving the height of the corona; but since it is probable that in 
the forms under examination the apical System added little or nothing to the height, 
this makes little practical difference, and previous authors have no doubt meant the 
corona when they have spoken of the test. 
The diameter of a scrobicule is the diameter of the scrobicular circle, and 
does not include the ring. 
The height of a plate is its greatest meridional diameter. The width of a plate is 
its greatest transverse diameter, parallel to the transverse sutures. 
Classification. — Fortunately no discussion of the broader divisions is necessary, 
since, with the exception of the doubtful Tiarechinus fragment, all the specimens 
appear to fall into either the Cidaridae or the primitive Ectobranchiata frequently 
grouped in a Suborder Diademina, of which the division into Families is as yet far 
from settled. It will therefore be most convenient to take the genera in the usually 
accepted systematic Order, and to introduce such remarks on their relationships as 
may be advisable under the discussion of each genus. The genera are Tiarechinus ? x , 
Triadocidaris, Anaulocidaris, Miocidaris, Plegiocidaris, Eodiadema, Mesodiadema, 
and Diademopsis. 
1 A note on the recently alleged affinity of Tiarecliiuus to the Arbaciidae will be given at the 
close of the memoir; see the Index. 
Resultate der wissenschaf'tl. Erforschung des Balatonsees. I. Bd. 1. T. Pal Anh. 
5 
