386 MISSISSIPPIAN BRACHIOPODA 



acters of the delthyrial plate and the syrinx. In all those species where 

 a sufficient number of individuals have been observed showing these 

 characters, they seem to be constant within reasonably narrow limits of 

 variation, and they have been assumed to be of good specific value in 

 other forms in which they have been observed sometimes in but a single 

 individual; the different characteristics of this plate are shown in its 

 transverse contour externally, whether flat, concave or longitudinally 

 keeled, .and in the length and width of the free extension of the syrinx. 



The earliest described species of the genus were S. textus and 8. carteri, 

 both described by Hall as members of the genus Spirifer beforfe the genus 

 Syringothyris was established by Winchell. 8. textus is a well-defined 

 species, but 8. carteri has always remained as more or less of an enigma, 

 and several of our Mississippi basin forms have been identified with it on 

 insufficient grounds; the true characters of the species still remain un- 

 certain and in the present report it has not been recognized. 



One reason for uncertainty in the identification of the various species 

 of Syringothyris is that two distinct generic types have commonly been 

 confused, the two types of shells being essentially alike in all respects 

 except that one has a delthyrial plate with a well-developed syrinx upon 

 its inner surface, while the other has the delthyrial plate with no in- 

 dication of a syrinx. This latter group of shells has been defined as a 

 new genus in this report and will be discussed later. 



SYRINGOTHYRIS EXTENUATUS (Hall) 

 Plate LXXII, Figs. 5-12 



1858. Spirifer extenuatus Hall, Geol. Iowa, vol. 1, pt. 2, p. 520, pi. 7, 



fig;. 6. 



1890. Syringothyris extenuata Schuchert, '9th Ann. Rep. N. Y. State Geol. 



for 1899, p. 33 (in part). 

 1900. Syringothyris extenuatus "Weller, Trans. St. Louis Acad. Sci., vol. 10, 



p. 77, pi. 1, figs. 1-3. 



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

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

 dimensions of a nearly perfect pedicle valve are: length from beak 

 to front margin 14 mm., height of cardinal area 19.4 mm., width along 

 hinge-line 47 mm., width of sinus in front 10 mm. The dimensions of a 

 nearly complete internal cast are : length of pedicle valve from beak to 

 front margin 14 mm., length of brachial valve 13.2 mm., height of cardinal 

 area 14.2 mm., width along hinge-line 40 mm., thickness 16.2 mm., width 

 of sinus in front 10 mm. 



Pedicle valve broadly subpyramidal in form, the surface sloping from 

 the beak to the cardinal extremities in nearly straight lines, or with a 

 slight convex or concave curvature, the lateral slopes very short from 



