ORTHID/. 239 
wide triangular fissure, is situated a strong tooth, while the dental plates partly encircle 
the saucer-shaped muscular cavity. On the outer side of these, and extending further 
down, are the large well-defined and margined ovarian spaces. The vascular trunks are 
also sometimes apparent. A large specimen measured— 
Length 16, width 19, depth 7 lines. 
Obs. In 1839 Mr. J. de C. Sowerby described his Orthis rustica as being “ Trans- 
versely oblong, rather square, depressed, uneven, with many rounded radii, which become 
more numerous towards the margin; front straight or slightly elevated. Length 1 inch 
1 line, width 1} inches. The hinge-area is triangular and rather large; radii between 
40 and 50. Loc. Wenlock; Valley of Woolhope.” ‘To this description was added the 
figure of a ventral valve only. I was, however, subsequently (in 1847) enabled to describe 
and fully illustrate the exterior and interior of this remarkable species from a magnificent 
series of perfectly preserved examples obtained by Mr. Lewis and myself near Walsall, 
during the excavation of the new Rushall Canal. I also picked up some very perfect 
valves, showing the interior, during my researches at Benthall Edge and Lincoln Hill. 
I must now again remind the student that one of the chief characters of Orthis rustica 
consists im its generally having a small interpolated rib between each two of the 
longer ones, or between those which extend directly from the extremity of the beaks to 
the margin; the number of ribs varying according to the age of the individual. 
It was from having overlooked this important circumstance that I was in 1847 and 
1848 tempted to separate, under the designations of rigida and Walsalliensis, two 
variations in form presented by this species, the first with fewer, the latter with 
smaller and more numerous ribs. Full-grown individuals of the typical O. rustica 
present about fifty-six ribs on each valve, while a full-grown example of O. Wa/lsalliensis 
would have upwards of 120 on each valve; and as the shape of the shell, though 
usually smaller, is similar to that of O. rustica, and the interior is likewise identical, 
we are reduced to consider it as nothing more than a variety of Sowerby’s species,— 
a conclusion I had arrived at soon after the publication of my paper in the ‘ Bulletin 
de la Société Géol. de France’ (1848): In the same year and subsequently Mr. 
Salter considered O. rustica, O. rigida, and O. Walsalliensis as distinct varieties of 
O. calligramma, Dalm.; but he added at the same time that he feared few students 
of Silurian fossils would feel disposed to go with him in uniting all the above- 
mentioned forms (and others, of which he gives a list) with O. calligramma. I regret 
to say that, although I entertain the highest possible opinion of Mr. Salter’s views 
relating to Silurian fossils, I cannot concur with him in the opinion above expressed, 
for were I to admit O. rustica and its var. Walsalliensis as varieties of Dalman’s species, 
I might find equal reason for admitting into it a far larger number of forms than Mr. 
Salter would like to see numbered among the varieties of O. calligramma. ‘The last-named 
shell has, in my humble opinion, been correctly considered by the generality of palaeon- 
tologists as distinct from O. rustica, both on account of its more oval shape as well as from 
