ON FOSSIL POLYZOA. 199 
Lonsdale, narrow and compact in the central portion of the zoarium, 
rather wider and widely separated as the margin is reached. Within the 
‘ ridges’ or ‘ bars,’ longitudinally, the cells are separated from each other 
by much thinner walls, and the apparently quadrangular character is now 
seen to be oval, but two oval cells touching each other at their proximal 
and distal extremities leave small angular spaces at the base, laterally, of 
each cell, p. 198, fig. 2, No.4. This angular portion is perforated, con- 
sequently each oval cell orifice is seen to be surrounded by four perforated 
angular spaces, or, in other words, the ‘quadrangular cells’ are really 
oval orifices in outline, with the angular corners perforated; and rows 
of these are separated by the raised ridges already referred to. 
II. Zoariwm in section. A transverse section of the zoariwm will show 
other points of structure referred to briefly by Lonsdale (p. 198, fig. 2, 
No. 2). We now find that the zoarium is raised in the middle, thinning out 
wedge-like towards the margins. In the centre the cells are perpendicular 
on each side of, what it will be convenient to call, the axial region. The 
cells on the right and left of the middle cells are slightly bent towards 
the right and left borders, and the angle of this bending of the cell 
decreases as the margin is approached. This shows the cause of that 
obliquity in the direction of the cells on either side of the central row, 
noticed by Lonsdale in the diagnosis of the genus Ptilodictya. He also 
observes that the ‘cells penetrate the coral obliquely.’ Superficially ex- 
amined this appears to be the fact, but the extreme outer portion of the 
surrounding ‘ cell’ orifice is not the true orifice of the cell; it is only the 
vestibule’(p. 198, fic. 2, No. 2,b). The true orifice is deeper down, and the 
area of the cell is comparatively small (ibid. a) when compared with the 
area of the vestibule. This will be better seen when I describe the longi- 
tudinal section. If a tangential section of Ptilodictya is made, we find 
crossing each area a bar, and at first sight this seems to be the homologue 
of the ‘tabule’ referred to by Professor Nicholson in his diagnosis of 
Heterodictya. They are not ‘tabule’ in P. Lonsdalei, but they are sec- 
tions of the cells which are reached at different depths, and they look, 
especially in transparent sections, very much like tabuls. 
IIT, Longitudinal sections. These I have made and studied very 
carefully, and some of my sections reach the cells at different depths. If 
the student will refer to fig. 2, No. 3, p. 198, a good general idea of the cell 
and vestibule may be obtained. In this drawing I have given the outline 
of ten cells (drawn from an opaque section), five on each side of the axial 
region, but the axis as shown in the drawing is never found so sharp in 
section as it appears to be in the figure. Here the cells are angular and 
sub-opposite, so that two cells are almost triangular ; the base of the 
triangle is the true orifices, and the apex is the proximal extremities of 
the cells, and the spaces left vacant, on either side, at the distal portion 
of the cells are the vestibules,! 
IV. The vestibules. In the transverse section (p. 198, fig. 2, No. 2) of 
the zoariwm of Ptilodictya, in fig. 2b, [have shaded the vestibules, while the 
cells are left white. The bars which separate the rows of cells are more 
deeply shaded, and when viewed in this aspect they appear to be club- 
like, decreasing in thickness as their extremities reach the cell. In some 
‘In thus alluding to the triangular character of the two cells, the student will 
understand my references better if the figures be reversed. 
