ORTHID A. 215 
to about half the length of the valve, and thus separates the two pair of muscular impres- 
sions ; in the interior of the ventral valve the prolongation of the dental plates circum- 
scribes a small muscular disc, with a mesial ridge along the centre. 
Length 5, width 53, depth 34 lines. 
Obs. This species would be very variable in its shapes if certain large specimens 
occasionally found in England and in Gothland were really referable to it. The typical 
form of O. hybrida, which occurs so abundantly in our Wenlock Shale and Limestone, is 
pretty constant, however, in its shapes, from the size of less than a pin’s head to that of about 
five lines and a half in length, and agrees in all essential particulars with figs. 15 and 16 of 
our plate. Dr. Lindstrém informs me that he has procured in the Island of Gothland 
specimens of an Orthis which he seems disposed to refer to the one under description ; 
it measured fully one inch in length by one inch two lines in breadth, and this 
would nearly agree with some similarly proportioned specimens (fig. 20) found near 
Walsall; but I am very uncertain whether either the Swedish or British examples in 
question do really belong to the same species as the small O. hydrida which is so abun- 
dant and characteristic a species in our Upper Silurian rocks. 
Position and Locality. At page 526 of the third edition of ‘ Siluria’ O. Aydrida is 
stated to occur in the Caradoc, Llandovery, and Wenlock formations; and the Survey 
Museum is said to possess specimens from the Lower Ludlow of Hillend, Martley, 
Abberley, and some other places ; but the Wenlock is the horizon where the shell is best 
known to me, and where, at any rate, itis the most abundant. In the Wenlock Limestone 
and its shales it occurs at Dudley, Hay Head, near Walsall, Buildwas, near Wenlock, 
&c. ; also, according to Messrs. Phillips and Salter, in various localities in the Malvern, 
Abberley, Woolhope, May Hill, and Usk districts; and again by Prof. Ramsay and 
Mr. Salter at Mathyrafal and Pen-y-Craig in the Lower Llandovery of North Wales. I 
have never met with any specimens in the Caradoc, nor am I acquainted with any 
examples from either Scotland or from Ireland, although it is said by Prof. M‘Coy to have 
been found in the last-named portion of the British Isles. 
Ortuis tunata, Sow. Pl. XXVIII, figs. 1—5. 
Oxtuis LUNATA, Sow. Sil. Syst., pl. v, fig. 15, 1839. 
—  orBIcuULARIS, Sow. Ibid., fig. 16. 
—  .uNata et, O. orpicuLarts, M‘Coy. Synopsis Sil. Foss. of Ireland, p. 32, 
1846. 
— — — Phillips and Salter. Mem, Geol. Survey, vol. ii, 
p. 290, 1848. 
—  orsicuLaris, F. Schmidt. Silurische Format. von Ehstland, &c., p. 213, 1858. 
—  wunata, Salter. Siluria, 2nd ed., p. 20, fig. 11, 1859. 
— — MCoy. Brit. Pal. Foss., p. 220, 1852. 
