138 MISSISSIPPIAN BRACHIOPODA 



of spine bases on and adjacent to the auriculations, and the very peculiar 

 development of the partition from the inner surface of the brachial valve 

 which so completely separates the visceral from the anteriorly produced 

 portion of the valve. The species is usually rather small, but examples 

 with a length of 30 mm. or more are not infrequent in some localities. 

 In most localities the fasciculate arrangement of the costaa is not a con- 

 spicuous feature of the species. 

 Horizon. Chester group. 



Genus ECHINOCONCHUS n. gen. 



Description. Shell productoid in form and with internal characters as 

 in the genus Productus, The external surface of the valves marked by 

 more or less sharply differentiated concentric bands which commonly 

 grow broader in passing from the beak to the outer margins, each band 

 bearing numerous, crowded, fine, appressed, imbricating spines, either 

 subequal or unequal in size, which are produced from elongate, node- 

 like bases. 



Remarks. This genus is proposed to include those shells which have 

 heretofore been placed in the genus Productus in the group typified by 

 P. punctatus. This whole group of species differs so fundamentally 

 from the members of the genus Productus as typified by the genotype, 

 P. semireticidatus, in their surface markings, that it has long seemed to 

 the writer that they should be separated under a different generic name, 

 although their internal characters are essentially alike. The genus is 

 represented in the Mississippian faunas under consideration by several 

 species, none of which suggest any intergradation with those of the 

 restricted Productus. 



ECHINOCONCHUS ALTERNATUS (Norwood and Pratten) 

 Plate XVII, Figs. 1-7 



1855. Productus alternatus Norwood and Pratten, Jour. Acad. Nat. Sci. 



Phil., (2), vol. 3, p. 20, pi. 2, figs. la-e. 



1858. Productus vittatus Hall, Geol. Iowa, vol. 1, pt. 2, p. 639. 

 1863. Productus gradatus Swallow, Trans. St. Louis Acad. Sci., vol. 2, 



p. 93. 

 1883. Productus alternatus Hall, Hep. N. Y. State Geol. for 1882, pi. 



(18) 49, fig. 14. 

 1883. Productus vittatus Hall, Hep. N. Y. State Geol. for 1882, pi. (18) 



49, figs. 15-17. 

 1892. Productus alternatus Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 



pi. 18, fig. 14. 

 1892. Productus vittatus Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, pi. 18, 



figs. 15 r 17. 

 1894. Productus vittatus Keyes, Mo. Geol. Surv., vol. 5, p. 43. 



Description. Shell large, usually as wide or wider than long, but 

 occasionally longer than wide, the hinge-line a little shorter than 



