246 BRITISH SILURIAN BRACHIOPODA. 
fifty or more small, sub-equal, radiating angular ribs, of which a certain number are due 
to interpolations, the ribs being separated from each other by concave interspaces of 
lesser width; the whole are crossed by fine concentric lines of growth. ‘Two specimens 
measured— 
Length 11, width 17 lines (Sowerby’s figured type.) 
a Pale 
Oés. With the very scanty and incomplete material at my command, I feel quite 
uncertain whether Mr. Salter was correct or not in considering O. plicata as a variety 
of O. calligramma. I therefore prefer following Messrs. Sowerby and M‘Coy in pro- 
visionally keeping the two under separate designations. One reason for this is the marked 
external difference in shape existing between Sowerby’s figure of O. plicata and 
that of Dalman’s O. calligramma; for while in the first (Pl. XX XVII, fig. 1) the 
hinge-line greatly exceeds the width of the shell, the reverse is the prevalent character 
in Dalman’s form. ‘Then, again, the much larger number of ribs in O. plicata, 
of which a portion are due to interpolation, if the shell so labelled by Salter in the 
Museum of Practical Geology (Pl. XXXVII, fig. 26) does really belong to the form 
under description. While describing O. plicata, M‘Coy remarks, at p. 222 of his work 
already named, that the outline varies considerably, as well as the degree of acuteness of 
the cardinal angles ; and Mr. Salter tells us that ‘‘ in the var. plicata the ribs are nume- 
rous, and, though regular and rounded, are frequently interlined ; that they are at least 
twice as numerous as in any of the previously described varieties of O. calligramma, vit. 
parva, calliptycha, virgata, and simplex, and that they show the cross striz, and fre- 
quently a longitudinal stria in the hollow between the ribs ;” but what somewhat puzzles 
me is that while Mr. Salter retains O. virgata and O. plicataas distinct varieties of O. calli- 
gramma, he should state, at p. 287 of the 2nd vol. of the ‘Memoirs of the Geological 
Survey, that “ O. virgata and Sp. ? plicatus, ‘ Sil. Syst.,’ are opposite valves of the same 
species.” Whether, therefore, O. vezgata should be considered as a synonym (or variety) 
of the form under description, or of O. calligramma, seems to be a question still open to 
some uncertainty ; but as its external form approaches most nearly to some varieties (with 
small and numerous ribs) of Dalman’s species, and as Mr. Salter informs us that its ribs are 
very regular and equal in size, we have provisionally, at any rate, allowed it to remain 
where placed by Salter, namely, among the varieties of calligramma. It would, however, 
be necessary, in order to be able to settle the question at issue, to seek for casts of the 
exterior of the so termed varieties virgata and plicata, for it is not possible to correctly 
judge as to the specific value of a new species from the mere inspection of the internal 
cast of one valve only, and this is all Sowerby was able to offer us in the present case. 
Position and Locality. O. plicata is said to be not a very rare shell in the Caradoe- 
Bala formation, and has been met with at Bryn-gwyn, and in several other places near 
Bala. Mr. Sowerby mentions also Goleugoed, Llandovery ; it was found also by Prof. 
Sedgwick in the Bala schists of Llansantfraid, Glyn Ceiriog, at Meifod, Mathyrafal, &e. 
