SUB-GENUS HETEROTRYPA. 153 



horizontal, and sometimes inosculating, are abundantly devel- 

 oped in both the axial and the circumferential portions of the 

 corallum. No marked difference exists in the tabulation of 

 the larger and smaller tubes. 



Obs. — The general form and external characters of M. Gir- 

 vmiensis are sufficiently noticed above ; but I wish here to 

 draw attention more closely to some points connected with its 

 microscopic structure. Tangential sections (fig. 29, d and e) 

 show very clearly that the corallum is composed of a uniformly 

 distributed series of large corallites, between which (but never 

 in more than a single row) are intercalated numerous variably 

 sized and angular small corallites. The large corallites are 

 peculiar in the complete distinctness of the wall of each from 

 the walls of its neighbours. Each tube Is surrounded with a 

 thick wall of its own, and being oval or circular, the points 

 of contact between adjoining tubes are very few ; while even 

 where the walls come together, there is absolutely no amalga- 

 mation between them. Nor do we find that the thick oval 

 or circular wall is a secondary development (as, for example, 

 in M. Treiitonensis, Nich.), since there are no signs of any 

 primitive polygonal wall to the tubes, as Is so commonly 

 the case in other species. Again, there. is the singular fea- 

 ture in tangential sections that there exist numerous well- 

 marked dark spots (the " Wandstrange " of Dybowski), either 

 in the substance of the walls of the tubes, or more com- 

 monly where two corallites come together. No clearly de- 

 fined lumen or central cavity can be detected (so far as I have 

 seen) in these, but they would appear to certainly represent 

 the hollow spines, or " spiniform corallites," of so many species 

 of Monticidipora, in an aborted condition. On the other hand, 

 the small corallites, which occupy all the Interstices left by the 

 large tubes, appear not to be bounded by definite walls, but 

 to be limited simply by the walls of the larger corallites. 

 Where, however, they abut against each other, instead of 

 against the large tubes, they are separated by thin partitions. 

 In tangential sections which pass at a little depth below the 



