THE HIGHLAND BORDER ROCKS OE THE ABERFOYLE DISTRICT. 
189 
Lingulella, Acrotreta, Acrothele, Siphonotreta, and possibly Schizambon. Acrotreta 
is by far the commonest genus, and several species are represented, the most abun- 
dant being Acrotreta nicholsoni, Dav. Next in relative abundance are the genera 
Lingulella and Acrothele. 
The identification of the specific forms belonging to each genus is often difficult, 
owing to the minute size of the fossils or to their imperfect state of preservation. 
The list gives the names of forms that can be determined with some approach to 
accuracy. 
Genus Obolus, Eichwald. 
More or less rounded Obolids have been found which appear to be referable to 
this genus, but it has not been possible to determine the species. Walcott* gives 
the range of this genus as Lower Cambrian to Lower Ordovician. 
Genus Lingulella, Salter. 
Lingulella aff. ferruginea , Salter. — The majority of the specimens of Lingulella 
found approach L. ferruginea in form and character. The shell is small, somewhat 
convex, and ovate in form. The front is rounded, the sides sub-parallel, tapering 
towards the beak. The median septum is seen in many specimens, and occasionally 
trades of muscle scars can be detected (figs. 20, 21, and 22). 
L. ferruginea has been recorded from the Middle and Upper Cambrian of Wales 
and the Upper Cambrian of Shropshire.! 
Lingulella aff. nicholsoni , Calloway. — A larger form of Lingulella is less common. 
That shown in fig. 23 is somewhat distorted and broken. It is the interior of the 
dorsal valve showing the median septum, from the end of which two lines appear 
to bifurcate. Traces of muscle scars can be detected in the specimen. This form 
approaches in shape and characters to L. nicholsoni , as described by Calloway.J 
The front and sides are rounded, and the margins of the shell are marked by fine 
concentric lines of growth. Calloway’s specimen is from the Shineton Shales of 
Shropshire. A similar form has been obtained from the Upper Cambrian of the 
Malvern Hills.§ 
Genus Acrothele, Linnarsson. 
Acrothele ( Obolella ) maculata, Salter. — Several specimens of this form have been 
obtained (figs. 1, 2, 3). Davidson || has remarked on the very transverse shape of 
the shell, and on the fact that it is often found much flattened. This is the largest 
brachiopod yet found in the Aberfoyle district. The best specimen (fig. 2) shows 
* Camb. Brack. (U.S. Geol. Survey Monog., vol. li, pt. 1), p. 378, 1912. 
t Davidson, Brit. Fossil Brack. (Pal. Soc.), vol. iii, pt. 7, p. 337 ; Matley, Quart. Journ. Geol. Soc., vol. lxvii, 
p. 301, 1911. 
J Quart. Journ. Geol. Soc., vol. xxxiii, p. 668, 1877. 
§ Ibid., vol. lviii, p. 141, 1902 ; Davidson, Sil. Brack. (Pal. Soc.), Suppl., p. 208, pi. xvii, figs. 31 and 32. 
|| Monog. Brit. Fossil Brack. (Pal. Soc.), vol. iii, p. 341. 
