242 BRITISH SILURIAN BRACHIOPODA. 
from sixteen to fifty simple, straight, radiating, rounded riblets, with concave interspaces, 
of variable width, between each pair of ribs; two or three very fine longitudinal lines 
may, by the aid of the lens, be seen in the interspaces left between the ribs, and these 
also are crossed by very fine concentric equidistant lines. In the interior of the dorsal 
valve a small cardinal process fills the centre of the triangular fissure; while a wide 
longitudinal ridge separates into two pairs the oval scars left by the adductor muscle : 
the diverging brachial processes or lamine are small. In the interior of the ventral 
valve the saucer-shaped muscular depression is of moderate size. ‘Two specimens 
measured— 
Length 10, width 10, depth 4 lines. 
Mo Agassi.) oO eg 
Obs. In 1827 Dalman described and figured his Orthis calligramma so as not to be 
misunderstood,’ and he distinctly intimates that the ribs are straight and simple; the 
shell was subsequently (in 1845) correctly described by M. de Verneuil, who had occa- 
sion to compare together a great number of Russian examples. No doubt it is variable, 
both in shape and in the lesser or greater number of simple ribs which orna- 
ment the surface of its valves in different specimens; it may therefore be convenient, 
perhaps, to maintain distinct varietal denominations for such forms as O. Davidsont, 
Vern., O. virgata, Sow., O. Scotica, M‘Coy, &c., for these shells present slight variations 
in their detail, while still preserving the essential character of the original or typical form; 
but I do not consider that we should be justified in uniting to it, as varieties, such shells 
as O. rustica, O. Walsalliensis, and others that are characterised, not only by a difference 
in general shape, but likewise by the presence of the numerous interpolated ribs, which 
do not exist in O. calligramma. he shell has also been well described by Prof. 
M ‘Coy, in his work on British Palzeozoic Fossils; and he justly observes therein, that 
“it is well distinguished from the old O. flabellulum of Sowerby (with which it has been 
sometimes confounded) ‘‘ by its more regular definite ribbing, and the fine longitudinal strice 
in the interspaces, without short irregular ribs, the sharp extension of the ribs more nearly 
to the beak on the casts, the greater inclination of the large cardinal area, and its being 
situated in the most convex instead of the flat valve.’ In many of the Russian specimens 
1 «Q, testa margine compressa, basi utrinque angulata, radiatim multisulcata et concinne striata, costis 
indivisis: valve majoris nate prominula subincurva. Locus: In calce cinerea Ostrogothiz, ad Skarpasen 
rarius, Dom. Olivecrona, Mus. Ac. Se. Holmiensis. 
“ Longit. 22 mm., latit. 24 mm., valve minoris basis latit. 21 mm. ; testa ]3 mm. crassa. 
“Facie accedit ad Orth. elegantulam, basalem et testudinariam, sed major, costis omnibus indivisis, 
et una cum sulcis longitudinaliter striatis. Ab. O. callacti facillime distinguitur costis multoties 
numerosioribus (circiter 32—34), itaque magis approximatis. Valva minor basi convexa, versus marginem 
applanata, ambitu plus quam semicirculari, basi rectilinea lataque, angulis lateralibus fere rectangulis. 
Valva major convexior, uate parum prominula sub-incurva, et ab ipsa basi parum remota. Margo cardinalis 
latus, sed brevis, levis, cum foraminis deltoidei brevisque vestigio. Sutura valvarum non nisi leviter 
undulata,” 
