376 bulletin: museum of comparative zoology. 



shell from the beak. The brachial valve shows a stout median sep- 

 tum which, according to Dr. Ha\Ties's drawings, is formed by the 

 fusion of two mutually convex septa each supporting the hinge-plate. 

 The median septum is divided, forming a large crural cavity, each 

 fork supporting a branch of the hinge-plate. The crural processes 

 seem to be thrown off from both the hinge-plate and the fork of the 

 median septum, so that each process is concave towards the pedicle- 

 valve. Npne of the dental lamellae, crural processes or median sep- 

 tum extends beyond one third the length of the shell from the beak. 

 The dimensions of a specimen of average size are as follows : — 



Length 26 mm. 



Width 16 mm. 



Thickness 14.5 mm. 



Angle between sides 57° 



Plications on pedicle valve 8 



Plications on brachial valve 9 



The species has been found in quantity in the Madison limestone 

 at Old Baldy Montana, and, by the writer, near Cherry Creek Basin. 

 Entire, free specimens are not common and frequently the shells are 

 crushed. The condition of the beaks is seen only in two very nearly 

 perfect specimens, but in these the pedicle-opening is obscured by 

 damage. Natural casts or moulds of the interior have not been 

 found, and what is known of the interior has been gained by a study 

 of sections drawn by Dr. W. P. Haynes while grinding down the beaks. 



This form cannot be mistaken for any other species in the collec- 

 tions from southwestern Montana except perhaps Camarophoria 

 ringcns (Swallow), identified by Girty from the Yellowstone National 

 Park. I quote from Swallow's description : — " sinus wide and shal- 

 low, containing about eight plications Surface marked with 



about fourteen large plications on each valve. Length 1.90; breadth, 

 1.43; thickness, .99" (Trans. Acad. sci. St. Louis, 1860, 1, p. 653). 

 Camarophoria ringcns is therefore a much broader form, with a sinus 

 and more plications than Axiodcancia platyplcura which has no sinus 

 at all. The small number of plications, the prominent median plica- 

 tion and furrow, the narrow triangular form and the flat sides are 

 sufKcient to distinguish this species from any other known form. 



