242 
PALAEONTOLOGY OF NEW YORK. 
Observations. The shells of this genus are all small, and the high, usually 
erect cardinal areas give them a distinct cyrtiniform aspect. Their essential 
characters are, however, very positively orthoid. In the typical forms of 
Orthis following the structure of 0. calligramma, there is, first the pyramidal 
form of the shell, less pronounced indeed than in Scenidium ; then, on the 
interior of the brachial valve, a general thickening and elevation of the delti- 
dial region as illustrated in Plate V, fig. 14. This thickening ends abruptly 
below the crural plates, and at the base of the narrow, simple, cardinal process, 
but a low median ridge is continued forward until it is merged into the radial 
plications of the shell. In the species described by Mr. Billings as 0. Merope, 
from the Trenton limestone,^ and a very closely allied, probably identical 
form from the Hudson River horizon at Cincinnati (in both of which the 
size and form is that of Scenidium), the interior features of the brachial valve 
are precisely of this character, and the shell evinces no progress from Orthis 
toward Scenidium except in its interior. A pronounced advance in these 
respects is seen in Scenidium Halli, Safford, from the Trenton horizon of 
Minnesota and Tennessee; here the deltidial area is more distinctly elevated 
and the median septum low, but sharply defined. In Scenidium pyramidale, Hall, 
of the Niagara, an abundant shell at Lockport, N. Y., and S. Lewisi, Davidson, 
of the Wenlock of England and Gotland, the septum of the brachial valve is 
higher, though not attaining so great a development as in the Lower Helder- 
berg species, S. insigne, where it extends for a considerable distance into the 
cavity of the pedicle-valve, virtually dividing it into two chambers. 
The extreme development of these characters is attained in Scenidium areola, 
Quenstedt (sp.), from the middle Devonian of the Eifelf and Torquay4 Here 
the cardinal platform is wholly separated from the bottom of the valve, and is 
supported by the median septum, the latter being a high vertical partition 
extending to the opposite valve and leaving only its subrostral cavity 
undivided. 
* Palaeozoic Fossils, vol. i, p. 139. 1862. 
t Kayser, Zeitschr. der deutsch. Geol. Gesellschaft, vol. 23, p. 612, pi. xiii, fig-. 5. 1871. 
| Davidson, British Devonian Brachiopoda. Snppl., p. 49. 1882. 
