BRACHIOPODA. 189 



1886. Orthis, Walcott. Bull. No. 30, U. S. Geological Survey, p. IIS. 



1886. Ortliis, Whitfield. Bull. American Museum Natural History, vol. i, No. 8, p. 300. 



1887. Orthis, PlatystropMa, Shaler. Mem. Kentucky Geological Survey, vol. i, pt. 3, pp. 18, 19, 22. 



1887. OiUns, Walcott. American Journal of Science, vol. xxxiv, p. 190. 



1888. Orthis, Hekkick. Bull. Denison University, vol. iii, p. 38 ; vol. iv, p. 14. 



1888. Orthis, Rwguebeeg. 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. 



1SS9. 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 costas, 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 ton 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 [deltidium 

 ampleotens), bound it on its lower side {deltidium sectans), 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 ti-iangular cavity beneath the beak, but in some genera, particulai'ly in Stropho- 

 MBNA, SpiRiFER and their allies, there is still another form of shelly plate which g-rows from the apex down- 

 ward, and to this the term pseudodeltidium was applied by Bronn. Among recent writei-s 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 delthyrium (deXra and dvpiov) is suggested. 



