MEXICAN BOUNDARY LINE. 149 



ARCOPAGIA. 



ARCOPAGIA TEXANA. 



Plate IV, Figure 3, a,h. 



Arcopagia Texana, Roemer, Kreide. von Texas, pi. VI, fig. 8. 



Orbicular, compressed, lentiform, inequivalve, subtortuous, cardinal margin nearly straight, 

 forming an obtuse angle with the posterior margin ; anterior muscular impression distinct, 

 elongated, linguiform; posterior impression suhrotund, approaching the cardinal margin; umbo 

 small and slightly prominent. 



CARDIUM, Lin. Lam. 



CARDIUM MEDIALE. 



Plate IV, Figure 4, a, b. 



Cordate, equilateral, venlricose; base profoundly and nearly regularly rounded; beaks promi- 

 nent ; posterior margin truncated, direct. 

 Locality. — 



CARDIUM CONGESTUM. 



Plate VI, Figure 5, a, h, c, d. 



Cordate, inflated, subequilateral ; umbo prominent ; beaks approximate ; ribs radiatino', 

 probably about twenty-five in number. 



An abundant species in the form of casts of entire specimens. A mere trace of the shell in 

 one of these leads to the inference that the ribs were carinated. It has some general resem- 

 blance to C. constantia, D'Orbigny, but, unlike that species, it is oblique. 



Locality. — Rio San Pedro. 



CARDIUM, Sub-genus PROTOCARDIA, Beyrich. 



The genus Protocardia of Beyrich was founded on the Cardium Eillanum of Sowerby. A 

 number of species have been figured by authors — four have been described in D'Orbigny's 

 PaliBontologie Fran9aise. They are generally indicated by concentric lines or ribs, and have 

 radiating strias only on the post-umbonal area. The hinge resembles that of the Linnjean 

 Cardium, and the species seem to pass into that genus, even through external characters. Never- 

 theless, they form a natural section, having no living representative, and characterizing the 

 cretaceous and older tertiary formations. 



CARDIUM (PROTOCARDIA) MULTISTRIATUM. 



Plate VI, Figure 4, a, b, c. 



Cardium mvltistriatum, Shumard. Geol. of Red River, p. 207, pi. IV, fif . 2. 



Subrotund, inflated, height and length nearly equal ; truncated posteriorly ; basal and anterior 

 margins rounded; surface of posterior submargin, with 14-15 regular radiating striae; remainder 

 of surface marked with fine, equal, rounded, close concentric stri(« ; summit rather prominent. 



Locality. — Leon Springs. 



