PHYLLIYORA. 
59 
lying in different directions)^ appear to influence the 
form of the teeth. 
Of the rows taken longitudinally I need not say 
much, it being more easy to explain the variations 
in the teeth when the rows are regarded transversely. 
Suffice it to mention that in the centre of the mem- 
brane there is a longitudinal row of teeth of dif- 
ferent form to any of the rest (called a rachis by 
Loven);” being, in fact, the symmetrical normal 
tooth from which all the others more or less sym- 
metrically diverge as the longitudinal series are 
placed farther and farther from the central one. 
From reference to an ideal vertical section of a 
plate with its tubercle (fig. 19.), it 
will be evident that on viewing the 
whole vertically (fig. 20.) through 
the microscope (the object being 
almost transparent), three outlines 
will generally be seen, that of the 
plate, that of the attachment of the tubercle to the 
Fig. 19 . 
Fig. 20 . 
plate, which I shall refer to as the 
base, and that of the free points of 
the tubercle, which I shall speak of 
as the apex ; the teeth will therefore 
be regarded as the plate and tubercle 
combined. 
The central plate and its tubercle differ from all 
the others on this membrane in being symmetrical. 
The plate is of a subquadrangular form, often some- 
what longer than broad, having its sides slightly 
hollowed out, and its ends nearly straight {Limax, 
fig. 21.), or with its anterior end (that nearest the 
