G. W. Lee — Trepostomata. 
145 
On the same grounds one could explain the absence or rarity of the tabulae in the 
axial region, or their complete absence in some species. It would be strange that 
organisms on the whole similar should have differed in such an essential respect as the 
complete absence of tabula? throughout their existence. It might then be suggested that 
the non-tabulated species were tabulated during a certain period of their existence, but 
had the faculty of re sorbing their tabulae as they grew. 
In connection with the suggestion put forward here, it is interesting to note a remark 
of Ulrich, that 1 “ In the Trepostomata and many Cryptostomata, the tubular zooecium 
really represents a series of superimposed cells.” Although this statement is not further 
elaborated it is nevertheless interesting as it seems to imply a conception which is not 
antagonistic to the one sketched here. 
Assuming that growth took place as suggested in the preceding remarks, we are led 
to the not improbable conception of a phylembryonic colony consisting of straight zooecia, 
thick-walled throughout. 
TV .— Classificatory Characters. 
Although this memoir is primarily descriptive, the remarks made in the preceding 
pages were necessary in. order to explain the course adopted in the selection of the 
classificatory characters on which it is here proposed to base the British Carboniferous 
species. 
Structure of the walls .—Ulrich and Bassler recognize two great divisions in 
Trepostomata Amalgamata and Trepostomata Integrata. The first includes those families 
in which the boundaries of adjacent zooecia are obscured by the more or less complete 
amalgamation of their walls, and the second the families in which the boundaries of 
adjacent zooecia are sharply defined by a black divisional line 2 . In the British fauna, so 
far as examined, a black line is occasionally exhibited in forms which are otherwise 
referable to the division Amalgamata. Its adoption as a classificatory character in the 
case of the materials considered here would lead to an inconsistent grouping of the species, 
and is therefore deferred pending further investigation. 
The purely morphological characters of the walls are readily observable and have 
been widely used in specific and generic diagnoses. That is, the thickness of the walls 
relatively to the size of the enclosed zooecial cavities is a tolerably constant character in a 
given species, while the shape of the wall as seen in long sections is often distinctive, 
B 2 
22239 
1 42, p. 322. 
2 45, pp. 15 and 40. 
