﻿ORTHIS. 



01 



pressure, so that it is difficult to determine the shell's exact form. When about in their 

 normal condition these casts are sub-orbicular, the length and width being nearly equal. 

 The dorsal valve is convex, and deeper than the opposite one ; muscular scars large and 

 in relief. The external surface of the shell appears to have been marked by fine, sub-equal, 

 radiating striae, which increase in number by numerous intercalations. The beak of the 

 ventral valve was incurved, the area of moderate width, and divided in the middle by a 

 pseudo-deltidium. Dorsal area narrow ; proportions variable ; length and breadth about 

 two and a half inches. 



In America 0. Mjymrionyx occurs in the Oriskany Sandstone, from which horizon, 

 perhaps, the Looe beds may not be far removed. It is found at Albany and Schoharie 

 Counties (New York) ; at Cumberland (Maryland), and other places. Prof. Schnur 

 figures it from the Grauwackenschichten of Priim, in the Eifel. 



Orthis interlineata. PI. XVII, figs. 18 — 23. 



Orthis interlineata, Sow. Geol. Trans., 2nd series, vol. v, pi. liii, fig. 11; pi. liv, 

 fig. 14, 1840. 



— — Phillips. Pal. Foss. of Cornwall, Devon, and West Somerset, 



p. 63, pi. xxvi, fig. 106, 1841. 



— parallela, Ibid. Pal. Foss., p. 64, pi. xxvi, fig. 109. 



Spec. Char. Shell transversely elliptical, sub-orbicular, or slightly rotundato-quadrate, 

 compressed ; hinge-line less than the width of the shell ; cardinal angles obtuse ; ventral 

 valve slightly convex, but deeper than the dorsal one ; area narrow ; deltidium small ; 

 beak inconspicuous ; dorsal valve very gently convex and depressed ; area linear. Surface 

 of both valves marked with numerous small, radiating, thread-like striae, which extend 

 from the beak to the margin, with wide interspaces between them, in the middle of which 

 a smaller rib commences at variable distances from the beaks and extends to the margin, 

 the whole interspaces being also marked by six or seven smaller striae. A few concentric 

 lines of growth are visible on both valves. In the interior of the ventral valve the teeth 

 are prominent, the dental plates extending to some distance along the bottom of the valve, 

 and forming an elongated oval depression with raised margin ; this cavity is divided in 

 the middle by an angular ridge ; the occlusor or adductor muscle forms a small, central, 

 oval scar, but not very apparent, while the larger elongated depressions on either side are 

 formed of two parts, the anterior or one close to the occlusor and lowest down being 

 due to the cardinal or divaricator, while the other, the posterior or lateral, which is parallel 

 but higher up, may have been formed by the ventral adjustor (?). In the interior of the 

 dorsal valve the cardinal process is situated between two projecting laminae (which perhaps 

 afforded attachments to the spiral arms). These laminae curve on either side and partly 



