78 Bulletin 15 388 



given rise to the name, Chalk Hills. All the specimens are in 

 the form of casts. The matrix is a consolidated, light grey clay, 

 very fine in texture. The casts show evidence of great pressure, 

 which, in many cases, has resulted in their distortion. With the 

 shells are beautifully preserved leaves 'of willow, birch and dicoty- 

 ledonous trees, showing that land was adjacent to the shallows in 

 which the deposition occurred. The material contains two new 

 species of Wmo and one of Anodonta. 



UNIO Philipsson 

 Unio ? trigoniaformis sp. nov., PI. 29, figs, i, 2 



Shell rather small, triangular, strikingly like Trigonia in 

 form. Posterior end somewhat pointed, anterior rounded. Basal 

 margin curved. Beaks large and prominent, placed at the anterior 

 third of the shell. Valves apparently thin and light, sculptured, 

 in the specimen described, by five, pronounced radial plications 

 with interspaces of nearly equal width. The number and position 

 of the plications varies in different individuals; in some they are 

 obsolete in the anterior area, and, in general, they tend to be moi^e 

 marked towards the posterior area. At the ventral edge of the 

 shell, the plications are prolonged beyond the interspaces, forming 

 a sinuous margin. Ivines of growth wavy, following the sinuosi- 

 ties of the base. Length of shell, 40 mm., height, 30 mm. 



Portions of the casts show a puncftate surface. This seems 

 to have been caused by a foreign growth on the outer surface of 

 the shell, forming encrustations analogous to those of Microcoleus 

 hyalinus on recent species of Unio and Anodotita. 



Unfortunately, no trace of teeth or of beak sculpture is to be 

 seen. 



This species is evidently not a member of the genus Ufiio, 

 sensu striHo and it is only referred to that genus until more mate- 

 rial is obtained by which the sedlion to which it belongs can be 

 definitely determined. That the species is a member of the Unio- 

 nidcB can hardly be questioned when the charadters of the shell 

 and the forms with which it is associatrd are considered. 



In some respecfts, this shell recalls the South American genus. 

 Cast alia. 



