310 MISSISS1PPIAN BRACHIOPODA 



the shell has reached one-third of its mature size the hinge-line has 

 reached nearly its maximum length, with a very narrow cardinal area. 

 During the more mature growth of the shell the length of the hinge-line 

 remains stationary, while the shell itself increases in length and the car- 

 dinal area increases in height. In rare instances the elongation of the 

 hinge-line is retarded still earlier in the life of the individual so that as 

 the shell increases in size the greatest width is not along the hinge-line, 

 but is in front of this line and the cardinal extremities become rounded in 

 the mature shell. 



This species is especially characteristic of the fauna of the Louisiana 

 limestone. In the original description of the species it was said to occur 

 also in the Chouteau limestone of central Missouri, and it has been fre- 

 quently identified by later authors from various Kinderhook localities 

 and horizons. All of these identifications, however, are believed to be in- 

 correct. Aside from in the Louisiana limestone, the species has been 

 recognized by the writer only in the yellow Kinderhook sandstone at 

 Kinderhook, Illinois. The specimens from this locality are all imperfect 

 and more or less poorly preserved. They seem to attain a somewhat 

 larger size than the Louisiana limestone specimens, and in none of them 

 has the form of the cardinal area been actually observed. The sand- 

 stone specimens exhibit the shallow sinus and low fold of the typical 

 members of the species, and the lines of growth seem to indicate the early 

 attainment of the maximum length of the hinge-line during the growth 

 of the shell, and for these reasons the specimens are provisionally referred 

 to this species. 



Horizon. Louisiana limestone. 



SPIRIFER VERNONENSIS Swallow 

 Plate XXXVII, Figs. 8-17 



3860. Spirifer vernonensis, Swallow, Trans. St. Louis Acad. Sci., vol. 1, 



p. 644. 



1865. Spirifer Vernonensis "Winchell, Proc. Acad. Nat. Sci. Phil., p. 119. 

 1909. Spirifer vernonensis Weller, Bull. Geol. Soc. Am., vol. 20, p. 302, 



pi. 13, figs. 3-8. 



Description. Shell of medium size, subsemicircular in outline, wider 

 than long, the greatest width along the hinge-line, cardinal extremities 

 angular. The dimensions of two individuals are: length of pedicle valve 

 23 mm. and 25 mm., length of brachial valve 17.5 mm. and 20.5 mm., width 

 31 mm. and 43 mm., thickness 19 mm. and 22 mm., height of cardinal area 

 3.5 mm. and 3.5 mm. 



Pedicle valve strongly convex, strongly arched from the beak to the 

 front, the median line often describing nearly a semicircle ; beak small, 

 pointed, strongly incurved; cardinal area of moderate height, arched, 



