64 AN ANNOTATED CATALOGUE 



to those now living. In America, unmistakable Uniones 

 appear in the Jurassic, or, if the determinations of Hall are 

 correct, may even date back to the Devonian. In the Lar- 

 amie beds of the northwest and in the strata of somewhat 

 later date there occur besides Unio, Plaiiorhis, and (7am- 

 peloma the following genera : Limnophysa, Gyraulus, 

 Physa, Balinus^ GoniobasiSy Vivipara, Valvata, and 

 some land forms. The conclusions presented would point 

 to a remote origin of the present facies of the molluscan 

 fauna now living in the area under consideration ; and with 

 the material now at hand important results await the biol- 

 ogist and palaeontologist in this field of investigation. 



The occurrence in Iowa of pearls in some of the fluvia- 

 tile bivalves has recently been made known ; the principal 

 localities being in Dallas county, where in the swift cur- 

 rent of the rapids of the Raccoon river below Van Meter are 

 found in abundance, Unio wardii Lea, and other species 

 rare elsewhere in the state. Of more than two hundred 

 specimens of Uniones collected here the majority contained 

 from three to a dozen or more pearls, some of which were 

 quite large ; the most of them, however, were rather small, 

 yet quite perfect and symmetrical. A few of those taken 

 from Unio wardii were of a delicate rose tint, while those 

 from U. gibbosus were dark purple. The other species af- 

 fording pearls were Unio rectus Lam., U. fragosus Con- 

 rad, U. cesopus Green, and Margaritana rugosa Barnes. 

 The habitat of these pearl-bearing species appears to be 

 very different from that referred to by Kuchenmeister who 

 found pearls most abundant in mollusks living in the still 

 parts of streams. Among the collections are a number of 

 monstrosities which present curious phases of molluscan 

 life. Many of these pathologic forms are evidently the re- 

 sult of injuries received ; but others are doubtless due to 

 unfavorable development early in life. Some of the most 



