SPIRIFER 331 



strongly convex from the cardinal to the anterior margins, somewhat 

 compressed towards the cardinal extremities, marked by plications 

 similar in form and number to those of the opposite valve. 



The minute surface markings of the shell are not preserved on the 

 specimens observed, but several concentric lines of growth of variable 

 strength are usually present, becoming stronger and more frequent 

 towards the anterior margin in mature shells. 



Remarks. This species is especially characterized by the strongly car- 

 mate mesial fold of the brachial valve, and when that portion of the 

 shell is preserved it is always easy of identification. In its surface char- 

 acters, especially the size and character of the plications, the species re- 

 semjbles 8. tenuicostatus, and worn, fragmentary examples of the two 

 species are sometimes indistinguishable. 8. lateralis, however, does not 

 possess the bifurcating plications nor the conspicuous lamellose, concentric 

 markings of 8. tenuicostatus, and the character of the fold and sinus, as 

 well as the general form and proportions of the valves, are totally different 

 in the two species, 8. lateralis being proportionally shorter with more 

 sharply angular cardinal extremities. 



The specimen which has been illustrated as 8. lateralis by Hall and Clarke, 1 

 is clearly not a member of this species, it has more the aspect of an under- 

 sized example of 8. logani, although the depth of the sinus seems to be too 

 great for an individual of that species of such a size. The shell which 

 Rowley has described as S. lateralis var. delicatus is scarcely worthy of 

 varietal designation, it being only a depauperate individual in the Spergen 

 Hill fauna of Indiana, where many species are represented by specimens 

 smaller than the more usual form of the same species elsewhere. 



Horizon. Warsaw formation and Salem limestone. 



SPIRIFER FORBESI Norwood and Pratten 

 Plate XLII, Figs. 1-3 ; Plate XLIII, Fig. 16 ; Plate LXXXIII, Figs. 1-2 



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



(2), vol. 2, p. 73, pi. 9, figs. 3a-c. 



1858. Spirifer forbesi Hall, Geol. Iowa, vol. 1, pt, 2, p. 600, pi. 13, fig. 1. 

 1894. Spirifera forbesi Keyes, Mo. Geol. Surv., vol. 5, p. 80, pi. 40, fig. 3. 

 1S95. Spirifer Forbesi Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 37, 



fig. 18. 



Description. Shell above medium size, much wider than long, the 

 greatest width along the hinge-line, the cardinal extremities acutely an- 

 gular. The dimensions of a pedicle valve of usual size are: length 

 24 mm., width along the hinge-line 54 mm., height of cardinal area 5 mm., 

 depth 10 mm. The dimensions of a larger individual with the valves in 

 articulation are : length of pedicle valve 27.3 mm., length of brachial valve 



l Pal. N. Y., vol. 8, pt. 2, pi. 32, figs. 1-3. (1894). 



