98 R. M. Brydone — Chalk Folyzoa. 



HOMALOSTEUA CAVKltNOSA , s]>. IIOV. PI. IV, FigS. 4-G. 



Zoarmm always free and uuilamiiiate, thick and strong. 



Zooecia diamond-shaped and convex, the outlines marked only by 

 liollows, no sutures being observable ; apertures in depressions at the 

 ])oints of the diamonds and circular except for a slightly flattened 

 lower end ; in the front wall there is a shallow chamber extending 

 from the centre of the front wall, where it communicates wdth the 

 surface bj^ a pore, to the edge of the aperture over which it opens by 

 a horizontal tubular opening, the upper edge of which when perfectly 

 preserved impinges slightly on the true outline of the aperture. These 

 openings are the only indication of the existence of this chamber in 

 a perfectly preserved specimen, but it is frequently exposed in 

 damaged specimens by the breaking away of its roof. 



Ooecia not observed. 



Avicidaria rare, vicarious, occurring as long narrow cavities with 

 rounded ends ; deep down in them there appears at the upper end 

 a narrow horizontal front wall which tapers away towards the middle 

 of the avicularium, but does not wholly disappear from sight until at 

 a point a little below the middle of the avicularium two slender 

 denticles spring from the edge of the avicularium at the general 

 zoarial surface-level ; these probably are the remains of a bar 

 spanning the avicularium, of which, however, no instance has yet 

 been observed ; below these denticles the side walls of the avicularium 

 are no longer horizontal but sloping steeply inward round the serai- 

 circular lower end. 



The species occurs from time to time at Trimingham. There is 

 a distinct suggestion of relationship between it and Mucronella (?) 

 Bathcri and M. (?) Spenceri in the front-wall chamber of the one and 

 the front-wall tubes of the others, which is, however, much weakened 

 by the presence of vicarious avicularia; and I have placed it under 

 Somalostega so as to keep it near H. suffulta, Marsson,' which seems 

 likely to be even more closely allied to it. 



HOMALOSTEGA VuLCANI, Sp. UOV. PI. IV, Fig. 7. 



Zoariam unilaminate, always encrusting. 



Zooecia bounded only by very faint hollows, no sutures being- 

 discernible, with slightly convex front walls and shoitish heel-shaped 

 apertures, which often have their lower lips slightly turned up ; they 

 are very freely sprinkled with small volcano-like eminences, some 

 proportion of which are shown to be avicularian by the preservation 

 of the mandibular cross-bar, which makes it probable that all the 

 others are also avicularian. These eminences show a strong tendency 

 to lie in a rough semicircle round the apertures along the hollows 

 which indicate the zooecial boundaries. 



Ocecia not observed. 



Avicidaria oi i^o sorts, (1) the accessory kind above mentioned, 

 (2) vicarious ; the latter are not infrequent, and have a strong 

 general resemblance to those of H. cavernosa, but their sides are not 

 straight but slightly bulging, and the front wall occupies all the 



' Die Bryo. d. Wcisa. Schrcibhr. cL Tnscin Pviigcn, p. 95, taf. x, fig. 4. 



