BRACHIOPODA. 
189 
1886. Orthis, Walcott. Bull. No. 30, U. S. Geological Survey, p. 119. 
1886. Orthis, Whitfield. Bull. American Museum Natural History, vol. i, No. 8, p. 300. 
1887. Orthis, Platystrophia, Shaler. Mem. Kentucky Geological Survey, vol. i, pt. 3, pp. 18, 19, 22. 
1887. Orthis, Walcott. American Journal of Science, vol. xxxiv, p. 190. 
1888. Orthis, Herrick. Bull. Denison University, vol. iii, p. 38; vol. iv, p. 14. 
1888. Orthis, Ringueberg. Proc. Acad. Nat. Sci. Phila., p. 134. 
1889. Orthis, Whitfield. Bull. American Museum Natural History, vol. ii, No. 2, p. 43. 
1889. Orthis, Nettelroth. Kentucky Fossil Shells, pp. 32-45. 
1889. Orthis, Beecher and Clarke. Mem. N. Y. State Museum, vol. i, No. 1, pp. 14-17. 
1889. Orthis, Simpson. Trans. American Philosophical Society, p. 437. 
1890. Orthis, Foerste. Proc. Boston Soc. Nat. Hist., vol. xxiv, p. 308. 
Diagnosis. Shell subcircular or subquadrate in outline. Valves more or less 
convex, the smaller or brachial valve being sometimes nearly flat or slightly 
concave. Hinge-line straight and equal to, or shorter than the greatest width 
of the shell. Cardinal area well developed on each valve and divided 
by an open triangular delthyrium.* Beaks more or less incurved. Surface 
covered by radiating costae, with faint evidence of median fold and sinus. 
In the interior of the pedicle-valve the large hinge-teeth are supported by 
dental plates which are more or less conspicuously developed, frequently 
resting upon the bottom of the valve. The bases of these are continued as 
a low elevation about the muscular area. The subdivisions of this muscular 
impression are rarely distinct; it is divided longitudinally by an inconspicuous 
median ridge and the larger expansions on each side were probably occupied 
by the diductor muscles; within these and lying close against the median 
ridge are the adductors, while the pedicle muscles covered the posterior 
deltidial portion of this area. 
In the interior of the brachial valve the apex of the deltidial cavity bears 
* The term deltidium was proposed by von Bdch for the triangular plate which, in many articulate 
genera, covers more or less completely the space between the outer margins of the dental ridges. This 
plate he describes as composed of two pieces which may either completely surround the foramen ( deltidivm 
amplectens), bound it on its lower side (deltidium sedans ), or the parts may be separated for their entire 
length by the foramen (deltidium discretum). These component parts of the deltidium take their origin 
from the margins of the triangular cavity beneath the beak, but in some genera, particularly in Stropho- 
mena, Spirifer and their allies, there is still another form of shelly plate which grows from the apex down¬ 
ward, and to this the term pseudodeltidium was applied by Bronn. Among recent writers there has been 
considerable laxity in the use of these terms and it is very doubtful if they can be applied with precision. 
In the discussion of mature characters with which this work has principally to deal, the term 
deltidium is applied to the outer plate covering the deltoid fissure, irrespective of valves, and for this so- 
called “deltidial fissure,” the term delthynum (deXra and Qvpiov) is suggested. 
