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Operculata. In the present instance, it arises from the inosculation of the 
tabulae with one another, giving rise to triangular and wedge-shaped or 
lenticular vesicles, hut not blister-like or sub-globular as in Cystiphylhtm 
and its allies. The wedge-shaped vesicles are usually marginal, short, and 
sometimes deep, the lenticular more often occupying the centre of a corallite. 
In every corallite in which these vesicular tabulae arc a feature, there is 
always a certain number of complete floors, and no case of a corallite’s 
visceral chamber tilled with the vesicular type alone has come under notice. 
In the Operculata in particular, the interior structure of the corallites is 
wholly composed of true endothecal vesicles. Such is the case in G onioph yllum, 
ffliizophyllum, and Arceopoma, whilst Calceola so far ditfers that the internal 
structure is dense, but at the same time very finely vesicular. In one species 
of Try plasma fl\ vermiformis) , vesicular tabulae have not been observed. 
In fractured or weathered horizontal sections, the peripheries of the 
complete tabulae often present a series of short depressions or nicks, which 
are the imprints of the septal spines resting on them. In three species — 
T. columnaris, T. derrengnllenensis, and T. delicatula — a very interesting 
feature lvas discovered in the presence of upstanding spines scattered over 
the upper surfaces of the floors; these are placed irregularly, or arranged in 
groups. In transverse sections prepared for the microscope, these appear as 
dark spots, or as dark rings with a refractive centre, and in either case often 
surrounded by a concentric secondary thickening. A similar structure w ? as 
observed by Lindstrom in his IPholidophyllum iubulaium. 
Oblique complete tabulae appear to be absent in T. vermiformis , and 
possibly in T. liliiformis also, but arc present in the other species ; rolling 
tabulae are met with in T. Lonsdalei, T. congregationis, T. columnaris, 
T. princeps, T. wellingtonensis, and occasionally in T. liliiformis-, typical 
concave floors are to he seen in T. Lonsdalei and T. liliiformis, and those 
with an upward convexity in T. congregationis. Marginal vesicular tabulae 
arc present in T. Lonsdalei, elongately lenticular in T. congregationis, both 
conditions in T. columnaris and T. princeps, and irregular lenticular vesicles 
in T. wellingtonensis and T. liliiformis. 
The wide tabulate non-septate area of Tryplasma again calls to mind 
the structure of the non-dissepimental genera Amplexus and Pycnostylus. 
15. Fossula . — I cannot detect any trace of a septal fossula in Tryplasma-, 
my observation is, therefore, in accord with Dr. Lindstrom’s description of 
Pholidophyllum. In the complete horizonta], oblique, or rolling tabulae of 
