PSEUDOSYRINX 411 



extremities, the umbonal region is rather strongly protuberant beyond 

 the hinge-line ; the mesial fold rounded on top, sharply defined and slightly 

 elevated at the beak, becoming broader and of moderate height anteriorly ; 

 the beak small, broadly pointed and rather strongly incurved; cardinal 

 area narrow, concave, lying at nearly a right angle to the area of the 

 opposite valve ; plications similar in number to those of the opposite valve 

 but much more sharply defined. 



The minute surface markings consist of exceedingly fine radiating striae. 

 Besides these, concentric lines of growth of greater or less strength are 

 present which are often crowded at intervals, especially towards the 

 front of the shell. 



Remarks. The specimen which has been selected as the holotype of this 

 species is a nearly complete silicified specimen whose locality is unknown. 

 Other less perfect examples from Warsaw, 111., and Crawfordsville, Ind., 

 have been studied. The species is characterized by its large size, the 

 nearly flat cardinal area which becomes arched just beneath the beak, 

 and by the small angle between the cardinal area and the plane of the 

 valve. Because of the silicification of the shell the minute surface mark- 

 ings are not preserved upon the type specimen, but they are shown on 

 the others and are best exhibited upon the Warsaw specimens. Upon 

 one Crawfordsville example the fine radiating striae appear on some 

 parts of the shell to be interrupted at short and rather regular intervals, 

 the intervals of interruption of adjacent striae being similar along con- 

 centric lines, the striae being alternate in position on either side of the 

 lines of interruption. This arrangement gives to the surface markings 

 an approach to the "twilled cloth" markings which are so characteristic 

 of Syringothyris and of at least some of the species of this genus. 



Besides the absence of a syrinx, this species differs from all the recog- 

 nized species of Syringothyris in its general proportions, and can not be 

 easily confused with any of them. There is a strong suspicion, however, 

 that the species may be the same as that to which Troost 1 gave the name 

 Cyrtia gigas. His description is entirely inadequate to allow the recog- 

 nition of his species in the absence of the type or of authentic specimens, 

 but the proportions of the cardinal area given by him are sufficiently near 

 those of the specimen used for the definition here published, although 

 Troost 's specimen was somewhat larger. In order to avoid confusion in 

 case Troost 's species and this one should ever be proven to be the same, 

 the same specific name has been used, a procedure which is allowable, 

 since Troost 's species has only been referred to the genera Cyrtia and 

 Syringothyris. 



Horizon. Keokuk limestone. 



16th Geol. Rep. Tenn., p. 12. (1841.) 



