244 BRITISH SILURIAN BRACHIOPODA. 
since the interior of M‘Coy’s species presents some well-marked differences, as may be seen 
in the figures above mentioned. O. plicata, Sow., has likewise been considered to be a 
variety of O. calligramma ; but as the ribs are numerous, and shorter ones are here and 
there interpolated between the longer ones, we will follow Prof. M‘Coy in retaining it, at 
least provisionally, as distinct. The variety Davidsoni (Pl. XXXYV, figs. 18, 19) was 
discovered for the first time in the Wenlock Shales near Walsall, by Mr. A. Lewis, and 
was by myself in 1847 referred to Dalman’s O. calligramma. In 1848, however, 
M. de Verneuil separated it completely from the form under description, adding, “This 
species has been confounded in certain collections with the O. calligramma from the 
Lower Silurian, from which it is distinguished by the development and small curvature of 
its area. In O. calligramma the beak, instead of inclining backwards, as in O. Davidsoni, 
is incurved, so as to place itself nearly in the plane of the longitudinal axis of the shell. 
We find in the United States, in the Trenton Limestone, which forms part of the Lower 
Silurian formation, a species, O. ¢triciuaria, Conrad, which is very close to ours, and from 
which it is distinguished by a larger number of ribs, and by the absence of a hinge-area 
in the ventral valve.” Mr. Salter, however, I think, correctly considers O. Davidsoni to 
be a simple variety of Dalman’s species, and so does D’Eichwald at. p. 827 of the 2nd 
vol. of his ‘ Lethzea Rossica.’ In the variety Davidsoni the ribs seem to vary from sixteen 
to nineteen in number, and the concentric equidistant striz are very apparent. It 
was found also in the Woolhope beds at Hope Quay, Minsterly, and in the Island of 
Gothland. 
Orthis Scotica (Orthisina Scotica, M‘Coy) (Pl. XX XV, figs. 20—22) seems to me to 
be also a variety of O. calligramma, and most certaimly does not belong to the genus 
Orthisina, with which it has been erroneously classed by M‘Coy. It is somewhat more 
subquadrate than are the ordinary forms of cal/igramma, with from twenty-four to twenty- 
eight simple ribs, and interspaces of about equal width. In the dorsal valve there exists 
a slight median depression ; but this is also observable in some specimens of O. call- 
gramma. Prof. M‘Coy states that in form it is intermediate between the O. zmflexa 
and O. plana (Pander, sp.), but is more depressed. ‘The fissure is, however, triangular and 
open, as in Orthis ; and when one can examine several specimens of this so-termed species, 
its passages into the common shapes of O. calligramma become quite apparent. It 
occurs with the ordinary forms of O. calligramma in the Caradoc Limestone at Craig 
Head Quarry, near Girvan, in Ayrshire; and also, according to Prof. M‘Coy, in the 
calcareous shales of Colmonel on the Stinchar. 
Before concluding our account of the many modifications presented by Dalman’s 
species, we must refer to a large variety (Pl. XXXV, fig. 8) which occurs in the Lower 
Llandovery rocks at Mulloch Hill Quarry, near Girvan, in Ayrshire, as well as at Cong, 
County Galway, in Ireland. This variety seems to have exceeded in size all the others, 
and to have presented very wide interspaces between its ribs. Orthis Thakel, var. 
convewa, of Salter, figured in Strachey’s ‘ Pal. of the Niti,” from the Northern Himalaya 
