Repoht of the State Geologist, 581 



nal area well developed on each valve and usually divided by an 



open triangular delthyrium. Beaks more or less incurved. 

 Surface covered by radiating costae or fine striae, with Paint 



evidence of median fold and sinus. 



In the interior of the pediole-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. This muscular impression is divided longitudi- 

 nally by an inconspicuous median ridge and the Larger expan- 

 sions 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 hears a more or less developed cardinal process or callosity, 

 which, in the typical group, is elongate and usually simple at its 

 termination. The dental sockets are deep and their inner walls 

 support short divergent crura. The muscular impression is more 

 or less distinctly quadruple, being divided longitudinally by a 

 median ridge extending forward from the cardinal process, and 

 transversely by fainter ridges. This quadruple scar marks the 

 place of attachment of the adductor muscle. 



Ramified vascular markings are often retained over the pallial 

 area, the principal trunks diverging from the median region and 

 nearly following the curve of the anterior margin of the shell. 

 Between these and the adductor impressions the markings of the 

 ovarian areas are sometimes visible. 



Shell-structure fibrous and impunctate in the typical group. 



This extensive genus has been divided into the following 

 groups or subgenera: 



Orthis, Dalman, sensu stricto. 

 I. Group of Orthis callactis, Dalman. 

 (Plate 9, Figs, 1-7.) 

 Shells plano-convex in contour ; costae, strong, sharp and com- 

 paratively few, rarely if ever bifurcating ; the cardinal area on 

 the pedicle-valve elevated and somewhat incurved ; dental lamellae 

 slightly developed, not extending the entire length of the 

 umbonal cavity. The cardinal process on the brachial valve is 



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