290 MISSISSIPPIAN BRACHIOPODA 



the lateral margins in the internal casts rounding regularly into the 

 lateral slopes of the shell; the delthyrium narrowly triangular with no 

 indication of deltidial plates in the casts, although such were probably 

 present ; each lateral slope marked by from six to eight, simple, rounded 

 or subangular plications which originate at the cardinal margin, be- 

 coming regularly less conspicuous laterally, the outermost ones some- 

 times becoming almost obsolete. Internally a strong median septum ex- 

 tends from the beak half way to the anterior margin, the dental plates 

 are short and join the median septum to form a spondylium, muscular 

 scars obscure. 



Brachial valve less convex than the pedicle, the convexity extending out 

 to the cardinal extremities and greatest near the middle ; mesial fold non- 

 plicate, rounded, of moderate width, bounded by deep rounded furrows, 

 only moderately elevated above the general surface of the valve ; each 

 lateral slope marked by from five to seven simple, rounded plications 

 entirely similar to those of the opposite valve. Internally the muscular 

 scars are obscure, the only marking shown upon the internal casts being 

 a slight median ridge which extends from the beak anteriorly to about 

 the middle of the valve ; the cardinal process is of moderate size and is 

 flanked by the diverging crural plates. 



Besides the plications the surface of each valve is marked by fine, 

 regular, sublamellose, imbricating, concentric lines of growth, about three 

 occupying the space of one millimeter. Shell structure thickly and rather 

 coarsely punctate, the perforations being arranged in the central portion 

 of the valves, and perhaps also laterally, in regular concentric lines cor- 

 responding with the depressions between the external markings of the 

 shell. 



Remarks. This species has been observed only as internal casts and 

 external moulds, from residual cherts at Springfield and elsewhere in 

 Missouri, the type specimens of the species being from that locality rather 

 than from Burlington, Iowa, as stated by the authors of the specie-s. The 

 proper horizon of the species is also incorrectly recorded in the original 

 description, the horizon as indicated by associated species in the cherts, 

 being as high as Keokuk rather than the Burlington. 



In its general form and surface markings the species resembles some 

 members of the genus Spiriferina much more closely than other members 

 of the genus Cyrtina. It especially resembles some shells from the Salem 

 limestone fauna which has been commonly identified as Spiriferina spinosa, 

 but it has a little less angular plications, and the cardinal margins of the 

 pedicle valve are not sharply defined, but round regularly from the 

 lateral slopes into the cardinal area. Internally fhe species is funda- 

 mentally different from Spiriferina, in the union of the dental plates in a 

 spondylium supported by a median septum. The punctate shell structure 



