382 MISSISSIPPIAN BRACHIOPODA 



mesial fold low, rounded, defined to the beak, smooth or sometimes faintly 

 marked by a median furrow ; lateral slopes convex, each divided into a 

 posterior and an anterior portion by a rounded ridge passing from the 

 beak to the point of greatest lateral extension of the shell, but the differ- 

 entiation is not so strongly marked as in the pedicle valve, the posterior 

 portion curves abruptly to the cardinal margin and is free from plica- 

 tions, the anterior portion curves more gently to the antero-lateral margin 

 and is marked by plications similar in form and number to those of the 

 opposite valve. 



The minute surface markings of the shell consist of very fine concentric 

 lines. Both valves also bear, at intervals, stronger concentric lines of 

 growth which traverse both the anterior and posterior portions of the 

 lateral slopes, and which terminate posteriorly in the pedicle valve at the 

 lateral margins of the true cardinal area. 



Remarks. This shell is one of the most sharply defined species in all 

 our Mississippian faunas and cannot be confused with any other. Ex- 

 amples of the shell here described as S. chouteauensis have sometimes been 

 identified as S. peculiaris, but that species has much more approximate 

 beaks with a longer hinge-line and lower cardinal area, and does not pos- 

 sess the conspicuous false cardinal area of S. peculiaris. The fold and 

 sinus in the two species are also different in character. These same char- 

 acters distinguish S. peculiaris from S. burlingtonensis, and in addition 

 the more anterior position of the greatest width of the shell in S. peculiaris, 



Horizon. Chouteau limestone of the Kinderhook. 



Genus CYRT1A Dalman 



Description. Shell small or of medium size, semipyramidal in form, the 

 greatest width along the hinge-line, the sinus of the pedicle valve and 

 the fold of the brachial valve well developed and non-plicate, the lateral 

 slopes of the valves plicated or not. The pedicle valve with a very high 

 cardinal area which may be flat or arched, the delthyrium narrowly tri- 

 angular and covered by a transversely convex pseudodeltidium ; inter- 

 nally the dental lamellae are conspicuously developed and may reach ante- 

 riorly beyond the middle of the valve; no median septum present. 

 Brachial valve as in Spirifer. Shell structure non-punctate. 



Remarks. The genus Cyrtia was originally established upon the species 

 C. exporrecta from the Silurian of Sweden, and one or more species in the 

 Silurian faunas of America are clearly congeneric with the genotype 

 from Sweden. The characters which serve to differentiate the genus from 

 Spirifer are its very high cardinal area, the strongly developed dental 

 lamella and the pseudodeltidial covering of the delthyrium. In general 

 form the members of the genus resemble Cyrtina more closely than Spirifer, 

 but they lack the spondylium and median septum of that genus, as well 

 as the punctate shell structure. 



