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The specimens collected by H. M. S. "Siboga" are from three different Stations, and 

 have a greenish-grey colour in the preserved condition. The outer lamina are only partly 

 removed in the younger specimens, much more so in the larger specimens. The size and 

 shape of the shells is rather different, radii are distinct in some specimens and hardly disting- 

 uishable in others; some have very small orifices, others much larger ones. But the shape and 

 structure of the opercular valves is the same or nearly the same in the different specimens ; 

 they closely resemble the figures and correspond to the description given by Darwix. 



The most typical specimens among those of H. M. S. "Siboga" were collected at 

 Station 152 during reef-exploration. According to the list, the depth at that Station was 32 m. 

 and the bottom was covered with Lithothamnium. The shape of these specimens is conical, 

 their colour greenish-grey, the exposed parietal tubes dark, nearly black. The orifice is irre- 

 gularly rounded and large, the sutures of the compartments are not quite distinct, much more 

 distinct, however, in one of the specimens, than in the others. A narrow but distinct radius 

 is represented almost at every suture in this specimen. The diameter of the largest specimen 

 was 38. 5 mm. at the basis; the thickness of the wall was here about 8 mm. The opercular 

 valves of one of these specimens were carefully cleaned, and found in perfect agreement with 

 Darwin's description, not considering little differences in colour, and in the degree in which the basi- 

 tergal angle of the scutum is rounded off, etc. In this specimen, the adductor ridge of the scutum 

 extends quite to the apex of the valve, as Darwin says it does in some specimens. lts 

 tergum has a distinct beak, the spur is quite close to the basi-scutal angle and its extremity 

 is pointed, perhaps not quite so sharply as Darwin says it is in one of the varieties, but 

 sharper certainly than in the two forms figured by Darwin. 



This form comes nearest to T. porosa var. (3) viridis of Darwin ; the inner surface 

 of the opercular valves, however, is not "clouded green" as Darwin said, but rather bluish- 

 black. These specimens were collected during reef-exploration at Wunch-bay, N.W.coast of 

 Waigeu-island [Station 152, August 12/13, JS99-] (Note. A sample of specimens collected during 

 the New-Guinea-Expedition of 1903 (February 13*) in the Bay of Mawi corresponds in most 

 respects to those of Station 152.) 



In the second place, there are specimens from Station 131. They are much smaller 

 than those from Waigeu-island and form two clusters of 7 to 9 specimens each. Their shape 

 is not conical, but some are depressed, others rather globular with relatively small orifices. 

 The surface is irregularly ribbed, the ribs more or less distinctly serrated near the base. The 

 outer lamina of the shell is quite preserved in some specimens, in most removed near the 

 orifice; the colour of the specimens (in preserved condition) is greyish-green and the parietal 

 tubes are blackish where exposed. Radii and sutures very indistinct. Size : largest diameter of 

 the basis 12 to 16 mm., height 7 to 9 mm. The opercular valves are much like those of the 

 specimens from Station 152, the scutum having the adductor ridge extending to the apex 

 of the valve, and the tergum being beaked and having the spur quite close to the basi-scutal 

 angle. The extremity of the spur, however, is more bluntly pointed. 



This form resembles T. serrata Darwin, to a certain extent ; on the other hand it is 

 so much like a true T. porosa that it seems impossible to separate it from that species. 



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