NO. 2209. NEW MViiSELS OF THE GENUS DIPLODON— MARSHALL. 383 



Nacre silvery white, tending to bluish white and iridescence pos- 

 teriorly. Right valve with one small and one large pscudocardinal 

 tooth, the latter grooved lengthwise; and a wingliko lateral tooth. 

 Left valve with a largo anterior pscudocardinal, a deeply sunken 

 linear mid-tooth, and a small posterior tooth. Lateral teeth of left 

 valve two, very strong. Anterior muscle scars deeply impressed, 

 posterior scars superficial. Anterior portion of shell much thickened, 

 especially between the pallial line and the ventral margin. Shell 

 gaping slightly front and back, enough to insert a slip of tliick paper. 

 Sinulus long, narrow, and pointed. 



The type. Cat. No. 270899, U.S.N.M., measures— length, 66 

 mm.; height, 37 mm.; diameter, 27 mm. It comes from Rio Negro, 

 Tacuarcmbo, Uruguay. It was presented to the museum several 

 years ago by Dr. Florcntino Felippone, and has been hero 

 under the name delodontus Lamarck. This species is related to 

 Dijdodon charruanus Orbigny, but is more nasute, having the ventral 

 margin nearly straight and more nearly parallel to the dorsal 

 margin. The pscudocardinal teeth of cJiarruanus are largely divided 

 into sharp points with corresponding sockets in the opposing teeth. 

 Compared with these the teeth of fortis are much more simple. 

 Authentic specimens of cJiarruanus, from Orbigny show that its beaks 

 are heavily sculptured and that this sculpture covers a considerable 

 area. The beaks oi fortis are eroded but the area eroded is loss than 

 the area sculptured in charruanus, and yet there is no indication of 

 sculpture. From this it may be inferred that the sculptured area in 

 fortis would be less than that in charruanus. Although of but mod- 

 erate size, its compact, chunky form, its weight, and the thickening 

 of the shelly matter, conveys an idea of strength and this has sug- 

 gested the specific name /oriis. 



THE TYPES OF SIX LITTLE KNOWN SPECIES OF SOUTH AMERICAN 



DIPLODON. 



The following six species were described by Mr. Charles T. Simp- 

 son in A Descriptive Catalogue of the Naiades, or Pearly Fresh-water 

 Mussels, published in 1914. His descriptions were based upon mate- 

 rial in the collection of the United States National Museum, but no 

 figures were given. The following descriptions and illustrations of 

 the types wiU help more clearly to define the various species: 



DIPLODON MIMUS Simpson. 



Plate 51, figs. 3-6. 

 Diplodon mimus Simpson, A Descriptive Catalogue of the Naiades or Pearly 

 Fresh-water Mussels, 1914, p. 1249. 



Shell small, solid, obliquely rhomboid, somewhat inflated along 

 the posterior ridge, wliich is low, rounded, and widely biangulate. 



