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them the frontal wall of the polj'pide-tube ascending towards the aperture and 

 which, contrary to the case in the Aspidostomidae and the majority of the Ste- 

 ganoporellidae, are always separated from the aperture, never seem to be developed 

 quite alike, although the difference between them may be greater or smaller in 

 the different species. To begin with, there is always a difference in size, and 

 further the smaller of them generally sinks more or less obliquely towards "the 

 corresponding lateral wall, so that it either does not reach the basal wall at all, 

 or only touches the latter to a smaller extent than the larger, the direction of 

 \vhich is more vertical to the base. In Th. mamillaris (PI. VI a, figs. 5 a— 5 e) 

 only one opesiula appears to be present. The two lateral recesses, formed by the 

 opesiular outgrowths, may in the different species be more or less completely 

 separated from the polj'pide-tube and the zooecial cavity, and in the case of 

 Th. Rozieri, var. labiata (PI. VI, figs. 6 a— 6 j) as well as in Th. novae hollandiae 

 (PI. VI a, figs. 3 a — 3 f) they form two almost completely closed spaces, which 

 however have a small foramen in their distal wall just within the distal margin 

 of the opesiula. While in these two forms the outgrowths join the basal wall in 

 a closed curved line springing from the lateral margin, the corresponding curved 

 lines in the other forms lack a greater or smaller part of the distal boundary, 

 as not only the distal wall of these lateral recesses, but also a greater or smaller 

 part of the lateral wall of the polypide-tube is absent (PI. Via, fig. 4b, PI. VI b, 

 figs. 1 e, 5 e, 6 b). These lateral recesses are most reduced in Th. Rozieri var. 

 californica (PI. VI b, fig. 2 d). A rare case in the genus Thalamoporella is found 

 in Th. Uoticha (PI. VI, fig. 7 i) and in some zooecia of Th. Harmeri (PI. VI c, fig. 

 1 f), the outgrowths in which, in the same way as in certain forms of Stegano- 

 porella, meet the basal wall in a continuous line, which takes up the entire bre- 

 adth of the wall, while the polypide-tube has at the same time a basal wall 

 of its own, and on the whole these opesiulse, in spite of the great variation they 

 show, may afford good specific characters. 



The structure of the distal wall is similar to that in Steganoporella, being 

 composed of a horizontal or slightly ascending basal part and a steeply ascending 

 short frontal part. As in the species Steganoporella a more or less well developed 

 oral shelf springs from their junction. 



The very peculiar calcareous spicules occurring in all the species of this genus 

 are situated partly in the cavity . itself of the zooecium, where the largest are 

 always found, partly in the space between the cryptocyst and the covering mem- 

 brane. They are found both in the avicularia and in the ooecia, and despite the 

 great variation they are subject to, they generally afford good specific characters. 

 The avicularia, which are as a rule smaller than the zooecia, though they may 



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