2 PROCEEDINGS OF THE NATIONAL MUSEUM vol.76 



Additional proof that they belong in this group of mollusks is found 

 in the general form. Their abundance indicates that the Reynosa 

 formation was of fresh-water origin. The land shell was probably 

 fossilized after being washed into the water, or perhaps it lived on 

 plants on the surface and dropped in and became fossilized. The 

 fossils are composed of chalky calcium carbonate. The matrix is of 

 the same kind of material, somewhat more compact, and in parts 

 composed of calcite in the form of small crystals. A few small pebbles, 

 probably of quartz, are included in the matrix. 



The literature relating to the Reynosa formation is quite extensive. 

 Two of the most recent papers are: 



1923. A. C. Trowbridge: A geological reconnaissance in the Gulf Coastal Plain 



of Texas, near the Rio Grande (U. S. Geological Survey, Professional 

 Paper 131-D, pp. 98-100). Regarding fossils he says (p. 100): "No 

 fossils have been found in the Reynosa formation except the remains 

 of land snails, crayfish, jack-rabbits, and a few other animals which 

 have become embedded in the surface as the limestone has been dis- 

 solved and redeposited, and except the fossils originally deposited in 

 the formations from which the gravel was derived." 



1924. Alexander Deussen: Geology of the Coastal Plain of Texas west of the 



Brazos River (U. S. Geological Survey, Professional Paper 126, pp. 

 102-108). He says (p. 103): "No fossils have been found in the 

 formation." 



Footnotes to these two papers give references to many other papers 

 relating to the subject. 



Three of the mussels and the land shell retain enough of their 

 features to warrant the descriptions given below. 



PLICONAIAS, new genus 



Shell subquadrate. Beaks with a number of wavy concentric 

 undulations. Posteriorly each undulation is completed by a fine 

 straight threadlike riblet running across the posterior dorsal area 

 toward the beak. Anteriorly the undulations nearly fade out but 

 are indistinctly completed by faint riblets curving toward the beak . 

 Posterior portion of the shell with several rude plications running 

 obliquely across the surface and of the pattern found in the plicate 

 North American naiades such as Amblema costata Rafinesque; 

 Megalonaias gigantea Barnes, Pledomerus trapezoides Lea. 



Type. — Pliconaias popenoei, new species, described below. 



Plicate naiades are found in the Upper Cretaceous formation of 

 Wyoming and Utah, but do not belong to this genus. 



PLICONAIAS POPENOEI, new species 



Plate 1, Figures 1, 9 



Shell subquadrate (posterior dorsal portion lost but evidently the 

 form of the shell approximated that of Amblema costata Rafinesque). 



