242 PALAEONTOLOGY OF NEW YORK. 



Observations. The shells of this genus are all small, ami 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, Saftbrd, 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 Torquay.:]: Here 

 the cardinal platform is wholly separated from the bottom of the valve, and is 

 supported l)y 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 Kaysbh. Zeitschr. der deutsch. Geol. Gesellschaft, vol. 23. ]k (512. pi. xiii. liir. fi. 1S7]. 



t Davidson, British Devoiiiiui Hi-.ii;liio)inda. Siipi)!.. p. 49. 1.SS2. 



