486 MISSISSIPPIAN BRACHIOPODA 



with a few individuals from another specific group from another locality 

 and horizon. The important fact is the central, average type of shell, and 

 as many such types as can be clearly recognized are worthy of specific 

 recognition. The gradation of these types into each other at their ex- 

 tremes is indication only of the close and probable genetic relationship 

 of the forms and probably of the youth of the generic type. With this 

 interpretation of the genus it will not infrequently be found to be difficult 

 or even impossible to identify specifically certain individual specimens, 

 and specific identifications will necessarily be made from groups of in- 

 dividuals. 



COMPOSITA TRINUCLEA (Hall) 



Plate LXXXI, Figs. 16-45 



1856. Tercbratula trinuclea Hall, Trans. Alb. Inst., vol. 4, p. 7. 

 1858. Terebratula trinuclea Hall, Geol. Iowa, vol. 1, pt. 2, p. 659, pi. 23, 

 figs. 4a-c, 5. 



1882. Athyris trinucleus Whitfield, Bull. Am. Mus. Nat. Hist., vol. 1, p. 



50, pi. 6, figs. 22-27. 



1883. Athyris trinucleus Hall, 12th Ann. Rep. Geol. Surv. Ind., p. 329, 



pi. 29, figs. 22-27. 

 1891. Athyris subquadrata "Whitfield, Am. N. Y. Acad. Sci., vol. 5, p. 585, 



pi. 14, figs. 1-3. 

 1894. Seminula trinuclea Hall and Clarke, Int. to Study of Brach., pt. 2, 



pi. 35, figs. 11, 12, 14. 

 1894. Sem.inula subquadrata Hall and Clarke, Int. to Study of Brach., 



pt. 2, pi. 35, figs. 13, 15. 



1894. Athyris trinuclea Keyes, Mo. Geol. Surv., vol. 5, p. 92. 



1895. Athyris subquadrata Whitfield, Geol. Surv. Ohio, vol. 7, p. 472, pi. 



10, figs. 1-3. 

 1895. Seminula trinuclea Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 47, 



figs. 5, 6, 10-14. 

 1895. Seminula subquadrata Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 



pi. 47, figs. 7-9, 15, 16 ; pi. 84, figs. 30-31. 

 1897. Athyris subquadrata Weller, Trans. N. Y. Acad. Sci., vol. 16, p. 258, 



pi. 18, fig. 16. 

 1903. Seminula subquadrata ? Girty, U. S. Geol. Survey, Prof. Paper, No. 



16, p. 296, pi. 1, fig. 5. 

 1906. Seminula trinuclea Beede, 30th Ann. Rep. Geol. Surv. Ind., p. 1322, 



pi. 22, figs. 22-27. 

 1909. Seminula subquadrata Bassler, Va. Geol. Surv., Bull. No. II A, pi. 



29, figs. 6-8. 

 1909. Seminula trinuclea Bassler, Va. Geol. Surv., Bull. No. II A, pi. 29, 



figs. 9-10. 

 1911. Seminula subquadrata Morse, Proc. Ohio State Acad. Sci., vol. 5, 



p. 383, figs. 13a-c. 



Description. Shell usually small, always below medium size, subovate 

 in outline, the larger specimens sometimes becoming subquadrate or sub- 

 pentagonal, the width usually less than the length, but sometimes equal 



