SPIRIFERINA 299 



In addition to the plications the surface of each valve is marked by 

 fine, regular, imbricating, sublamellose lines, about four of which occupy 

 the space of one millimeter. The shell substance is finely punctate, the 

 perforations having a tendency to be arranged in concentric lines, al- 

 though this arrangement is not perfectly carried out. 



Remarks. This species may be easily recognized by the great width of 

 the shell and by the mesial plication in the fold and sinus. In this latter 

 character it resembles S. solidirostris from a much lower horizon in the 

 Mississippian, and occasionally an individual with less extended hinge- 

 line than common, rather closely resembles the earlier species, but or- 

 dinarily 8. transversa has somewhat finer plications than 8. solidirostris, 

 and a narrower umbonal region. The chief variation shown by different 

 individuals of the species is in the proportional width of the shell. The 

 specimen whose dimensions have been given is an entirely normal indi- 

 vidual, but occasionally examples occur whose length is fully two-thirds 

 the width, but they possess the characters of the species in all other 

 respects and are united with the normal form by intermediate variations. 



Horizon. Chester group. 



SPIRIFERINA SPINOSA (Norwood and Pratten) 



Plate XXXV, Figs. 50-58 



1855. Spirifer spinosus Norwood and Pratten, Jour. Acad. Nat. Sci., Phil., 



(2) vol. 3, p. 71, pi. 9, figs. la-d. 

 1858. Spirifer spinosus Hall, Geol. Iowa, vol. 1, pt. 2, p. 706, pi. 27, figs. 



5a-c. 

 1883. Spiriferina spinosa Hall, Kep. N. Y. State Geol. for 1882, pi. (35) 



60, figs. 26-29. 

 1894. Spiriferina spinosa Hall and Clarke, Int. to Study of Brach., pt. 2, 



pi. 31, figs. 4-7. 



1894. Spiriferina spinosa Keyes, Mo. Geol. Surv., vol. 5, p. 85. 



1895. Spiriferina spinosa Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 35. 



figs. 26-29. 



Description. Shell below medium size, broader than long, the greatest 

 width along the hinge-line or a little anterior to it, the cardinal extremi- 

 ties angular or a little rounded. The dimensions of two perfect speci- 

 mens are : length of pedicle valve 14.2 mm. and 12.5 mm., length of 

 brachial valve 11.5 mm. and 10.3 mm., width 18.7 mm. and 20.4 mm., thickr 

 ness 12.5 mm. and 9.8 mm., height of cardinal area 2.6 mm. and 2.4 mm., 

 width of mesial sinus in front 6.1 mm. and 5.3 mm. 



Pedicle valve most convex posterior to the middle, the umbo prominent, 

 the surface curving abruptly to the cardinal margin and more gently to 

 the anterior margin, often a little compressed towards the cardinal ex- 

 tremities ; the mesial sinus sharply defined, originating at the beak, of 

 moderate width and rather deep, subangular in the bottom; the beak 



