.Vo/c.s" on Riclnnond attd Related Fossilfi 47 



apjiearance of an anterior lobe, but this part of the margin 

 of tlic shell is not ])reserved in the specimen at hand. 



This specimen is imperfect, and exhibits only the interior 

 of the shell. The figure here presented consists of a cast of 

 this interior. No muscular or vascular markings are discern- 

 ii)le. The species occurs in the Whitewater member of the 

 Richmond grouj), and the type was found by John Misener, 

 at l^ichmond, Indiana. 



Dinorthis retrorsa, Salter 



Plate II, Figs. 1 A, B, C; Plate III, Figs. 4, 4a, 4c 

 Orihis retrorsa, Salter, was described (in the Memoirs of 

 the Geological Survey of Great Britain, vol. 2, pt. 1, 1848, 

 ]). 3^73, pi. XXVn, figs. 3, 4) by Salter as a variety of hi.s 

 Orthis inflata. His original description follows: 



Ventral valve gibbous, the center rather raised. Dorsal flat, broadly 

 depressed along the middle, edge not recurved; beak suppressed; area at an 

 obtuse angle with the valve. 



Locality. — Bird's-hill Quarries, North of Llandilo, in limestone. 



In this description the terms ventral and dorsal are used 



in the same sense as that in which they were employed by 



Hall in volume I of the Paleontology of New York, published 



in 1847, and familiar to American readers. Later these terms 



were employed by paleontologists in a directly opposite sense, 



and more recently the terms pedicel and brachial have come 



into use, so that a revised version of the original description 



of OrfJiis retrorsa would read: 



Brachial valve gibbous, the center rather raised. Pedicel valve flat, 

 broadly depressed along the middle, edge not recurved; beak suppressed; 

 area at an obtuse angle with the valve. 



Two sets of figures, differing distinctly in outline, accom- 

 pany the original description, and are reproduced with the origi- 

 nal numbering on plate III of this journal. Figure 3 evidently 

 represents the brachial valve of a species of Ilebertella while 

 figure 4 represents the pedicel valve of a species belonging to 

 the Plaesiomijs division of Dinorthis. The term retrorsa refers 

 to the retrorsion of the beak of the pedicel valve, and hence 

 the specimen represented by figure 4 is regarded as the type. 



