220 BRITISH SILURIAN BRACHIOPODA. 
in length, the one was four lines in depth while the other assumed almost double that 
proportion ; again, the greatest breadth generally occurs at the anterior half of the shell, 
while in other specimens it is situated about the middle. ‘The beaks of both valves 
also come so near together in some examples that the areas are hardly apparent. The 
identification of this species has occasioned me some trouble, and is still somewhat uncer- 
tain. While studying Sir R. Murchison’s original specimens of Atrypa polygramma 
(figs. 1, 2, of our plate), im the Museum of the Geological Society, I at once perceived that 
the shell so named belonged to the genus Or¢dis, and bore great resemblance to simi- 
larly shaped shells which occur in the Wenlock Shales of the Pentland Hills. These 
latter Mr. Salter had already alluded to, in the Appendix to Mr. Geikie’s memoir on the 
Silurian rocks of the Pentland Hills, as at first appearance bearing much resemblance to 
O. resupinata, and as looking like a magnified O. reversa, but hardly belonging to that 
species. Murchison’s specimens of 4. polygramma agree entirely as to size, shape, and 
sculpture, with others found in the Pentland Hills, and, as was justly observed, bear 
much resemblance to some forms of O. resupinata and O. Michelini, but the Silurian shell 
is easily distinguished from those two Carboniferous forms by the total absence of those 
slender spmes which rise from the strie in the two species above named. It is, however, 
possible that the Irish type of O. reversa may be a smaller variety of O. polygramma ; 
but in that case I question whether the Scottish shell from the Ayrshire beds, which 
has likewise been referred to Salter’s Irish species, is really the same thing. All reference 
to O. polygramma was omitted in the second edition of ‘ Siluria,’ but has been added to 
the third, published in 1867. 
Position and Locality. I am uncertain with reference to the age of the rock forma- 
tion from which Murchison’s two examples of 4. polygramma were obtained. In the 
‘Silurian System’ the rock is stated to be Caradoc, and the locality Powis Castle Park ; 
but since 1839 a portion of the Caradoc has been transferred to the Llandovery, and it is, 
therefore, possible that O. polygramma may belong to the last-named group. In Scotland 
the shell occurs in the Wenlock Shales of the Pentland Hills, where it is common, but 
mostly in the condition of well-preserved internal casts. 
Ortuis REvERSA, Salter. Pl. XXIX, figs. 11, 12, 18. 
Ortuts REVERSA, Salter. Addenda to the Synopsis of the Silurian Fossils of Ireland, 
p- 72, pl. v, fig. 2, 1846. 
Spec. Char. Round, about as wide as long, or slightly transverse with age. Hinge- 
line usually very short. Ventral valve moderately and evenly convex. Dorsal valve 
convex, with a shallow depression or sinus near the front; front edge slightly raised ; 
beak small, incurved ; area very narrow. Surface of both valves marked by numerous 
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