BRACHIOPODA. 189 



1886. Orthis, Walcott. Bull. No. HO, U. S. Geologic'il Survey, p. 119. 



1886. Ort/iis. Whitfield. Bull. Aiiioi-ioaii Museum Niituval History, vol. i, No. 8, \i. 300. 



1887 Orthh. Platystrophia, Shaleh. Meiu. Kentucky Geolofjital Survey, vol. i, pt. 3, pp. 18, 19, 22. 



1887. Orthin, Walcott. American Journal of Sc ience, vol. xxxiv, p. 190. 



1888. Ortliis, Herrick. Bull. DeniKon Univeivity, vol. iii, p. 38; vol. iv, p. 14. 



1888. Orthis, Ringdeberu. Proc. Acad. Nat. Sci. Phila., p. 134. 



1889. Orthis, Whitfield. Bull. American Museum Natural History, vol. ii. No. 2, p. 43. 

 1889. Orthis, Nkttelroth. Kentucky Fossil Shells, pp. 32-45. 



1889. Ort?iis, Bbecher 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, Foerstb. 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 ihe 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. 



Iji 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 -wsls proposed by von Boch for the triangular plate -which, in many articulate 



genera, covers more or less completely the space between the outer margins of the dental i-idges. This 

 plate he desci-ibes as composed of two pieces which may either completely surround the foramen (deltidium 

 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 oi-igin 

 from the margins of the triangular cavity beneath the beak, Viut in some geneva, particularly in Stropho- 

 MENA, Spirifer and their allies, thei-e is still another form of shelly plate which grows from the apex down- 

 ward, and to this the term pseudodtltidiitm was applied by Bkonn. Among recent writers there has been 

 considei-able laxity in the use of these terms and it is very doubtful if they can be applied with pi-ecision. 

 In the discussion of mature characters with which this work has principally to deal, the tei'm 

 deltidium is applied to the outer plate covering the deltoid tissure, iri'espective of valves, and for this so- 

 called " deltidial tissure," the term delthyrium (SikTa and Ovpiov) is suggested. 



