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This curious little shell seems to throw light on the function of 
the callus in Acrothyra and Acrotreta. fn ordinary species of Acro- 
treta the strong thickened ring around the foramen, within the shell, 
only needs to be raised still further to produce a siphon. And the 
siphon in this species, attached as it is to the dorsal side, holds the 
position of the callus in Acrothyra. 
This must be near in age to Acrotreta (jemma of Billing’s, than 
which it is a little larger, but as we do not know anything of the 
interior of Billing’s species (which belongs to the Arenig horizon) we 
do not use his name. 
LePTOHOLUS cf. LINOULOIDES. 
A small linguloid shell is not rare in this shale. As in others of 
this genus, the umbo of the ventral is weak and short, and so the two 
valves are not easily distinguishable. Owing to the thinness of the 
valves the internal features are only faintly indicated. The ventral 
shows two lateral ridges diverging from the umbo, and a callus is 
obscurely indicated ; one example shows a trace of a vascular trunk 
on one side. The dorsal has an obscure medium septum extending to 
the middle of the valve. 
Sculpture . — A very fine concentric striation is visible on some 
valves. 
Size . — Usual length, .3 mm.; (largest, 3^ mm.); width, 2| mm.; 
(largest, 2| mm.) 
This species is nearly as large as Liugulella lincjuloides of the 
Lower Paradoxides beds near St. John, a species which we would also 
refer to Leptobolus. The outline also is similar, but the umbo of the 
ventral is weaker ; this and the smaller size may be due to a more 
pelagic habitat. 
Linoulella cf. Davisii, McCoy. 
Examples of a Linguella, which though smaller than the above 
species of the Lingula Flags and Tremadoc slates of Britain, has the 
same general form, are found in the Cape Breton beds. It has the 
nearly straight base and sub-parallel sides of McCoy’s species. The dor- 
sal valve has on the interior a median septum two-fifths of its lengthj 
