LOWER CARBONIFEROUS BRYOZOA 
Fig.31 Rhombopora hexagona sp. nov. Line drawings of external features; 
a, colony form; scale bar = 1 mm; b, autozooecial apertures surrounded 
by heterostyles in hexagonal arrangement, scale bar = 0.1 mm. 
0.30 
| 
ria ee 
| 0.10 i aan 
: O ADI 
| ¢ AD2 
mm 0.00 
: ie2eomd 5) 6 7) 6584) 3) 21 
Reverse Obverse Reverse 
Autozooecial rows 
ja 
0.40 
: 0.30 
0.20 | 
Oo \wil 
¢ IWwt2 
mm 0!0 
———< ——— Se 
1072 Sa 4s) 6) 7) wan) 4e5e2.) 
Reverse Obverse Reverse 
Autozooecial rows 
b 
ig. 32 Rhombopora hexagona sp. nov. Graphs of mean values of 
| aperture size and spacing; a, apertural size; b, apertural spacing. (For 
explanation of AD1, AD2, IWT1 and IWT2 see Fig. 18). 
133 
in an interlocking hexagonal to pentagonal pattern. These hexagons 
range in size from 0.66 x 0.25mm to 0.23 x 0.20mm, with the 
greatest dimensions occurring on reverse surfaces. Autozooecial 
apertures are generally situated distally within these areas. 
Hemisepta are common and are of two types.A prominent superior 
hemiseptum occurs within the endozone four-fifths along the cham- 
ber on distal walls (Fig. 29). They are thin, short (0.04—0.06mm) and 
have a similar skeletal structure to chamber walls. At the exozone the 
proximal chamber walls bend through 30° and thicken rapidly to form 
robust inferior hemisepta 0.13 long by 0.04mm thick. These have a 
sharp pointed distal extremity and bend marginally into the vestibules. 
They are composed of laminated skeleton in which lamellae are 
orientated parallel to the zoarial surface. 
Table 8 Measurements of Rhombopora hexagona (in mm). N=19. 
NM x Mn Mx CVw CVb 
Jé\D) 135 0.63 0.48 0.92 4.62 Uh 
Z2 25 5.23 4 6 9.93 WS 
AD1 145 0.16 0.10 0.26 20.94 8.32 
AD2 146 0.09 0.04 0.18 15.57 5.13 
IWT1 139 0.26 0.12 0.66 30.34 3.74 
IWT2 136 0.17 0.07 0.39 27.71 4.37 
AH 14 0.01 0.01 0.01 50.00 - 
AW 31 0.01 0.01 0.02 16.04 - 
ET 21 0.43 0.32 0.50 3.1/2 8.75 
TE 42 0.08 0.03 0.15 17.37 2.96 
DISCUSSION. Rhombopora hexagona 1s only the second species of 
the taxon, after R. cylindrica, to be recorded from Carboniferous 
strata of the British Isles and has previously been noted from 
Courceyan strata of Hook Head, County Wexford (Dresser 1960 
MS).A previously recorded species R. radialis Owen, 1966 from the 
Viséan of Derbyshire is regarded as being an arthrostylid rather than 
a rhomboporid and is reassigned to the genus Pseudonematopora. 
Rhombopora hexagona is easily recognised externally from its 
cylindrical branches on which autozooecial apertures of varying 
dimensions (which is unusual) are surrounded by a hexagonal 
pattern of small heterostyles, and internally by the possession of two 
hemisepta of different sizes and a thin exozone. 
The relationship between autozooecial apertural diameter and 
apertural spacing is illustrated graphically in Fig. 32. Where aper- 
tures are long (high AD1) and thin (low AD2) longitudinal 
autozooecial spacing is moderate (low-high WT 1), and autozooecial 
spacing between adjacent rows is great (high IWT2). Where aper- 
tures are short and fat (low ADI values; high AD2 values), 
autozooecial spacing tends to be moderate and narrow (moderate 
IWT1 values; low IWT2 values). There is an inverse correlation 
between autozooecial apertural diameters AD1 and AD2 (Fig. 32a) 
and a moderately positive correlation between autozooecial apertural 
spacing IWT1 and IWT2 (Fig. 32b). In one specimen (C on Tables 
9-12) where 14 autozooecial rows, as against the mean of 10, are 
developed, these correlations are not good. 
Dimensions of 56 other Carboniferous Rhombopora taxa are 
tabulated below. R. hexagona differs sufficiently from them, both 
morphologically and dimensionally, to justify its erection as a new 
species. It most closely resembles R. attenuata Ulrich 1890, in 
which two acanthostyle types are present, and R. gracilis Ulrich 
1890, in which acanthostyles are developed at interapertural wall 
junctions only, but differs in acanthostyle development as well as in 
the size and spacing of autozooecial apertures. 
STRATIGRAPHICAL RANGE. 
Asbian). 
Lower Carboniferous (Courceyan— 
