R. M. Brydone—New Chalk Polysoa, etc. Tih 
width of area seem fairly representative. The specimen shown. in 
Fig. 4, though very exceptional in its large and graceful avicularia, 
quite 99 per cent. of specimens conforming closely with the type, 
raises an interesting question whether Fig. 3 ought to be regarded as 
the true standard and Fig. 4 as a sport, or whether Fig. 4 ought to 
be regarded as the true standard and Fig. 3 as due to mechanical 
gauses—attrition and solution—or to the general prevalence of some 
local condition unfavourable to a full development of the avicularia. 
As the two specimens grew contemporaneously and under identical 
conditions, so far as can now be judged, the question is more acute 
than in such a case as Cribrilina Gregoryt, where the avicularia 
are mere rings. until we get high up in the zone of Act. guadratus, 
where they become mandibular. 
Mempranipora Brirannica, var. prR&cuRsOR, nov. Pl. VIII, Fig. 5 
This is the form which occurs in the Upper Senonian, especially in 
the zones of Act. quadratus and Belemnitella mucronata. The zocecia 
are distinctly shallower, longer, and wider than in the type, the walls 
of adjacent cells are practically always distinguishable, and the 
avicularia lie practically on their backs and along the surface of 
the zoarium and with their long axes nearly parallel to those of 
the zocecia. The ocecia are so fragile that I have not yet seen one 
preserved. Length of area 48 to -64mm., width ‘36 to -40 mm. 
Mempranipora Britannica, var. DeMIssa, nov. Pl. VIII, Fig. 6. 
This form occurs not unfrequently at Trimingham. It is inter- 
mediate between the type and the var. precursor in that its avicularia 
lie along the surface of the zoarium, with their long axes nearly 
parallel to those of the zoccia, but distinctly on their sides. It is 
clearly distinguished from both by its pyriform zocecia with consider- 
able extent of front wall. The type does, it is true, often show a 
local tendency towards a pyriform zocecium with front wall, but this 
always appears to be due either to crowding, which forced the 
zocecium to grow some distance before it could open out its area, or 
to the failure of the preceding zocecium to produce an av icularium’ or 
an ocecium to occupy the space provided for it; the tendency does not 
go farther than may be seen here and there in Fig. 4. In the 
var. demissa the pyriform shape is obviously a fundamental character 
(which might be held to justify a specific separation), and is developed 
concurrently with the provision for the avicularium of the preceding 
zocecium. Besides these specific points of difference there is a con- 
siderable difference in matters of degree which is incapable of any 
exact definition, but which makes the variety readily recognizable 
with a pocket magnifier. Length of area -36 mm., width ‘32 mm. 
EXPLANATION OF PLATE VIII. 
Fic. 1. Membranipora invigilata, Chalk of Trimimgham. x 20. 
;, 2. Ditto, another specimen. Ditto. x 20. 
ir Ore Membr anipora Britannica, type- Syseuaee. Ditto. x 20. 
», 4. Ditto, another specimen. Ditto. x 20. 
Woes, Ditto, var. precursor. Zone ot Actinocamax saan atus, Wanchiesiar. x20. 
BB 
; Ditto, var. demissa.- Chalk of Trimingham. «x 20. 
