216 



Some Developmental Stages of Orthothetes Mixutus X. Si\ 



By Edgar R. Cumings. 



The specimens discussed in this paper are from the abandoned quarry 

 known as the Cleveland Stone Company's quarry, located one mile north 

 of Harrodsburg, Monroe County, Indiana. This quarry is in the so-called 

 Bedford limestone, and the specimens come from the top of the quarry — 

 and also from near the summit of the formation. They are, so far as I 

 can ascertain without having seen the original specimens of Hall.* specifi- 

 cally the same as Spergen hill forms referred by the latter gentleman to 

 the Orthis (Terebratulites) umbraculum of Schlotheim.t 



Description of the shell: 



Shell semiovate to subquadrate in old individuals; hinge line usually 

 less than the greatest Avidth of the shell, especially in young individuals; 

 cardinal extremities forming an obtuse, or sometimes a right angle with 

 the lateral margins. Surface finely plicated; plications increasing toward 

 the mai-gin by interstitial addition. Crests of the plications crenulated by 

 equally spaced fine concentric lines. 



Ventral valve concave with a pronounced tendency to irregular growth 

 about the beak. In mature individuals the beak becomes strongly retrorse 

 and is greatly elevated, equaling in height one-half the length of the shell. 

 Area well defined, flat, showing in well preserved specimens a low ridge 

 on each side of the prominent deltidium and parallel with its margins. 

 The younger specimens seem to show a perforation at the apex of the 

 deltidium. 



Dorsal valve regularly convex, greatest elevation about one-third of 

 the way from the beak to the front margin, though there is considerable 

 variation in this respect in shells of different age. Usually some flattening 

 at the cardinal extremities. Area very narrow or usually scarcely at all 

 conspicuous. 



Interior of the ventral valve showing rather prominent teeth, which 

 diverge -nidely. Cardinal process in the dorsal valve elevated, projecting 



*Acting under the impression that some of the original specimens of Hall were in the 

 Albany Museum, the writer sent a number of specimens of the form under cons'deration to 

 Dr. John M. Clarke to compare with Hall's specimens. 'While owing to the fact that Hall's 

 specimens are not at Albany, Dr. Clarke could not make the comparison, nevertheless he 

 gives it as his opinion that the two are probably identical. 



tPetrefk. I, 2-56; Schnurr, Brachiop. der Eifel 216; Bronn Lethsea Geog. I, 361. See 

 Hall, Trans. Alb. Inst. 4, p. 12. 



