167 
G. W. Lee — Trepostomata. 
growth often co-exist in the same branch, apparently without obeying any definite order 
of succession. There are 40 apertures along a distance of 1 cm. They are as a rule 
sub-polygonal, their partitions being generally thin, but sometimes they are sub-circular, 
when the margins are thicker. An acanthopore can generally be seen at the junction- 
angles. Maculas of small thick-walled cells are common. 
(b.) Internal structure. Longitudinal sections are highly characteristic: the peri¬ 
pheral end of the walls is typically moniliform. The swellings are very numerous— 
numbering up to 20 in large specimens—and are separated by pronounced constrictions. 
The tabulae are few in the axial region, but numerous in the peripheral region, where 
they are close-set, being separated by intervals which can be less than their own diameter. 
They bear a central reniform perforation, which is bordered by a raised rim pointing 
towards the proximal end of the tubes. In some instances the perforation is covered by 
a convex plate, also situated on its inner or proximal side. Two of these peculiar bodies 
are shown in the first and fifth tubes from the top, pi. xv., fig. 22. The ratio of axial 
region to diameter is 0'5 : 1 in cylindrical branches. 
Tangential sections show two sets of acanthopores : comparatively large ones are 
deep seated, and are restricted to the junction-angles, while a dozen small ones encircle 
the zooecia in shallow sections. 
The reniform shape of the perforation of the tabulae is well shown on a polished 
section belonging to the Nicholson Collection, where it can be seen that the perforation 
is not orientated in any definite direction. 
Distribution .—The holotype—and many paratypes preserved in the Nicholson Col¬ 
lection—are from Redesdale, Northumberland, presumably from the shale associated with 
the Ironstone, where both the hollow and solid forms co-exist. Small specimens referable 
to T. howsei were collected by Mr. J. Rhodes, 1 mile E. of Waterhead [R. 3431] ; at 
Hirsthead Sike, Liddel Water, Cumberland [R. 3278] ; and at Chatton Quarry, Chatton 
Moor, Northumberland [R. 4586], the last being from the Woodend Limestone, pre¬ 
sumably. In Scotland large examples were obtained from the top of the Calciferous 
Sandstone at Arbigland, Kirkcudbrightshire, and from the position of the Lawston Linn 
Limestone, a quarter of a mile N.W. of Grilnockie Tower, Dumfriesshire. It is 
interesting to note that the above localities may in a broad way be said to lie on 
the same horizon, approximately near the middle of tbe Visean. Remarkably large 
examples were collected from a black limestone—probably the Poolvash Limestone—at 
Port St. Mary, Isle of Man, by Messrs. J. Smith, R. Dunlop and J. Wright. Another 
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