330 MISSISSIPPIAN BRACHIOPODA 



centric lamellose lines of, growth which are so characteristic of 8. tenui- 

 costatus. 



Horizon. Keokuk limestone, Warsaw formation and Salem limestone. 



SPIRIFER LATERALIS Hall 

 Plate XLIII, Figs. 11-15 



1858. Spirifer lateralis Hall, Geol. Iowa, vol. 1, pt. 2, p. 661, pi. 23, figs. 



7a-c. 

 1901. Spirifer lateralis var. delicatus Rowley, Greene's Cont. to Ind. Pal., 



vol. 1, p. 68, pi. 23, figs. 21-23. 

 1906. Spirifer lateralis delicatus Beede, 30th Ann. Rep. Geol. Surv. Ind , 



p. 1314, pi. 20, figs. 3-3a. 



Description. Shell of medium size, much wider than long, the greatest 

 width along the hinge-line, the cardinal extremities angular. The dimen- 

 sions of a complete example, the holotype of the species, are : length 

 20.5 mm., width 51 mm., thickness 16 mm., height of cardinal area 4 mm. 



Pedicle valve with its greatest convexity near the middle, the umbonal 

 region broad and rather compressed, the surface curving strongly from 

 the beak to the anterior margin and very gently in a lateral direction, 

 becoming a little compressed towards the cardinal extremities ; beak 

 compressed, obtusely pointed, incurved; cardinal area arched, the curva- 

 ture becoming greater towards the beak, the inferior portion directed 

 posteriorly nearly in the plane of the valve, the lateral margins very 

 sharply defined, sloping with a gently convex curve from the beak to the 

 cardinal extremities; lateral slopes each marked by 25 or more fine, 

 rounded, simple plications, most of which terminate posteriorly along the 

 cardinal margin, towards the cardinal extremities the plications become 

 faint and almost obsolete; mesial sinus originating in the umbonal 

 region, in the posterior half of the valve it is shallow and ill-defined, 

 anteriorly it becomes deeper but is still ill-defined, at the anterior margin 

 it is produced in a tongue-like extension, it is marked by plications 

 which are similar in size to those upon the lateral slopes but which in- 

 crease by division in passing from thn beak to the anterior margin, about 

 15 being present in front. 



Brachial valve suboqually convex with the pedicle, the greatest con- 

 vexity near the front margin; mesial fold not sharply defined, strongly 

 elevated and carinate posteriorly, becoming more rounded towards the 

 front, posteriorly where the fold is carinate along the median line of the 

 valve, the sides of the fold are concave and curve gradually into the 

 lateral slopes of the valve, anteriorly the sides become convex, the fold 

 is occupied by about 16 rounded plications which originate through the 

 division of probably a single one at the beak ; lateral slopes of the valve 



