190 
PROFESSOR T. J. JEHU AND DR ROBERT CAMPBELL ON 
the exterior of the dorsal valve, with the ventral valve appearing from underneath 
due to displacement, and with the pedicle foramen appearing above. Apparently 
this species hitherto has been recorded in Britain only from the Middle Cambrian 
of Wales. 
Acrothele ( Redlichella ) granulata, Linn. — The exterior of the ventral valve is 
shown in fig. 5, and with a strong lens the granulation on the surface, which is typical 
of this species, can be seen. A drawing of this surface ornamentation is given as an 
inset to fig. 5. This form has been recorded from the Middle Cambrian of Shrop- 
shire,* Warwickshire, f and from the Lower Ordovician of the South of Scotland. \ 
Acrothele aff. coriacea, Linn. — In the specimen figured (fig. 4) we see the rough 
external surface of the ventral valve, and a strong lens makes it clear that the rough- 
ness is due to minute granulation and fine, slightly elevated striae, as described by 
Walcott. § It has been found in the Cambrian Rocks of Scandinavia, and Walcott 
gives the range of this species as Middle Cambrian to the Passage Beds between the 
Cambrian and Ordovician Systems. 
G-enus Acrotreta, Kutorga. 
Acrotreta nicholsoni, Dav. (figs. 7-12). — Specimens of this species are the most 
numerous by far of all the forms obtained in the cherty beds of the Aberfoyle district. 
The ventral valve has a conical or pyramidal form, with the circular pedicle opening 
at the apex. The posterior margin is triangular in shape and grooved, forming a 
pseudo-deltidium. The dorsal valve is relatively more depressed in form. Fine 
concentric striations are seen by -the aid of a lens on the external surface of the shell. 
This species has been recorded from the Shineton Shales of South Shropshire and 
the Bonsil Shales of the Malvern Hills. || It is common in the Arenig and Llandeilo 
Beds of the South of Scotland, and extends also into the Caradoc Formation.|||a 
Acrotreta socialis, von Seebach (figs. 13-15). — Fig. 14 shows an exceptionally 
well-preserved example. It is the interior of the dorsal valve, and the position of 
the median septum, the central muscle scars and the cardinal scars can be discerned. 
In another specimen the interior of the ventral valve is shown with a well-marked 
apical callosity (fig. 15). This species occurs in the Middle Cambrian of Sweden and 
Denmark,** and very similar forms have been recorded from the Middle Cambrian of 
Comley (Shropshire) ft and the Upper Cambrian of the Malvern Hills. || 
Acrotreta ( Obolella ) all. sabrinee, Calloway. — The specimen, of which a photograph 
is given (fig. 16), is the interior of the dorsal valve. The photograph is not good; 
the characters are better displayed in the actual specimen. The shell is somewhat 
* Matley, Quart. Journ. Geol. Soc., vol. lxvii, p. 302, 1911. 
t Lapworth, Proc. Geol. Soc., vol. xv, p. 346, 1888-89. 
+ The Silurian Rocks of Britain (Mem. Geol. Sur.), vol. i, p. 675, 1899. 
§ Walcott, (Jamb. Brack. (U.S. Geol. Survey Monog., vol. li, pt. 1), p. 642, 1912. || Ibid., p. 696. 
IT The Silurian Rocks of Britain (Mem. Geol. Survey), vol. i, p. 675. 
** Walcott, Camb. Brack. (U.S. Geol. Survey Monog., vol. li, pt. 1), p. 713, 1912. 
ff Matley, Quart. Jour. Geol. Soc., vol. lxvii, p. 303, 1911. |f Ibid., vol. Iviii, p. 144, 1902. 
