98 hk. M. Brydone—New Chalk Polyzoa. 
thin, but maintains accurately the circular areal outline. This makes 
it impossible to confuse the base of the damaged ocecium with the 
internal front wall of an avicularium as the latter sets well back from 
the areal outline. 
Avicularva vicarious, resembling very closely those of IM. subacumi- 
nata, but a little larger in proportion to the zocecia and with the nodal 
points standing out conspicuously. 
I have only one specimen, from Studland, of this species; but 
specimens of any adherent Polyzoa are so exceedingly scarce in the 
Studland Chalk and the species throws such useful light on the 
preceding one that I have felt justified in disregarding the general 
objection to a species founded on a single specimen. 
MemBranrrora DEMIssA, noy. (Pl. VI, Fig. 5.) 
Syn. M. Britannica, var. demissa, Bryd., GEOL. MaGc., 1910, p. 77, 
Pl. VII, Fig. 6. 
I am convinced that I was wrong in ignoring the doubts I felt at 
the time and treating this form as a variety of JL Britannica. 
Subsequent experience shows clearly that the front wall habitually 
developed in IL, Britannica solely as a platform for the ocecium or 
avicularium of the preceding zocecium never approaches in size or 
systematic nature that of JL demissa, while the bold sub-triangular 
areas of If, demissa are also quite distinctive. The specimen which 
I now figure—from Studland, of all unlikely places—is practically 
the only one I possess which shows clearly and perfectly the very 
fragile ocecia. There are six perfect examples in the Figure. They 
are merely gentle and vaguely outlined swellings, with free edges. 
which are marked off by a faint constriction and coincide exactly with 
the normal areal outline, and they form a strong contrast to those of 
M. Britannica, which are very bold. J. demissa first appears in rare 
small forms in the base of the zone of B. mucronata, but is represented, 
by normal forms as soon as the higher Chalk of the Isle of Wight and 
Studland is reached. 
The same treatment should be accorded to the other form described 
at the same time as a variety (var. precursor) of M. Britannica, and 
this form must stand as a new species, I. precursor. It is now 
known to range down to the zone of MM. cor-testudinarium, and 
though upwards it ranges into the zone of B. mucronata, it does not 
appear to reach the Weybourne Chalk, and as UZ. Britannica does 
not appear to range down into the Weybourne Chalk, they never 
even meet. It would be interesting to know the exact range of | 
Reptoflustrella Meudonensis, D’Orb.,1 which looks like a relation of 
M. Britannica, though clearly distinguished by its avicularia. 
Mempranipora Woopwarot, Bryd.,? var. PINGUESCENS, nov. 
CBT Vil, ie N62) 
This adherent form, which I know only from Trimingham, deserves. 
a recognition which seems properly limited to varietal. The round- 
ness of its areas and ocecia, the slenderness and flatness of its. 
1 Pal. Terr. Crét. Franc., vol. v, p. 572, pl. 731, figs. 19-21. 
? GEOL. MAG., 1910, p. 258, Pl. XXI, Figs. 1-3. 
