RH1PIDOMELLA 155 



are nearly uniform in size and increase by bifurcation and intercalation, 

 they are crossed by concentric lines of growth which vary in strength and 

 position on different individuals. The shell substance is pierced by 

 radiating canals or tubules of two sizes, which follow the direction of the 

 costae, the larger ones open at more or less distant intervals along the 

 summits of the costae, and openings of the finer ones are closely crowded 

 throughout the entire intervening space, giving to the shell when in 

 proper condition of preservation a distinctly punctate appearance. 



Remarks. This species differs from R. burlingtonensis in its somewhat 

 smaller size, its finer costae, its more elongate outline, and its more distinct 

 mesial sinus of the brachial valve. Besides these external differences, 

 the cardinal process is not so prominent as in R. burlingtonensis, and the 

 socket plates are more distinctly produced into crura-like extensions than 

 has been observed in that species. 



The species occurs near the summit of the Kinderhook and base of the 

 Burlington limestone, occurring rather commonly in the white cherts of 

 lower Burlington age at Louisiana, Missouri, which is the type locality. 

 The species was originally described by Rowley from immature specimens 

 about 6 mm. in length, the larger examples present in the same fauna 

 being identified as R. burlingtonensis. A careful examination of speci- 

 mens corresponding with the type of the species in size and in stage of 

 development, and also of mature examples, has led the writer to the con- 

 clusion that the small shells are certainly the young of the larger ones, 

 although the smaller examples are proportionally somewhat broader than 

 the mature ones. There seems to be, however, sufficient reason for the 

 separation of these shells from R. burlingtonensis, and consequently the 

 name R. diminutiva must be extended to cover the mature as well as the 

 younger representatives of the species. 



Horizon. Upper Kinderhook and lower Burlington limestone. 



RHIPIDOMELLA THIEMEI (White) 

 Plate XXI, Figs. 8-27 



1880. Orthis thiemei White, Contr. to Pal., No. 8, p. 164, pi. 41, figs. 4a-d. 

 1892. Orthis Thiemii Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, pi. 6 A, 



figs. 14-15. 

 1901. Rhipidomella burlingtonensis Weller, Trans. St. Louis Acad. Sci., 



vol. 11, p. 181, pi. 16, fig. 6. 

 1904. Rhipidomella thiemei Girty, U. S. Geol. Survey, Prof. Paper No. 21, 



p. 48, pi. 10, figs. 1-2. 



Description. Shell small, subquadrate to subovate in outline, the length 

 and width subequal or wider than long, the hinge-line much shorter than 

 the greatest width, the cardinal extremities rounded. The dimensions of 

 an undersized but very complete specimen are : length of pedicle valve 

 7.5 mm., length of brachial valve 7.4 mm., greatest width 8.4 mm., length 



