white.] ANNOTATED CATALOGUE. 25 



sentatives now living in the waters of the Mississippi drainage system. 

 These types of living Uniones, which have such well-defined represent- 

 atives amoug the fossil species of the Laramie Group, are among those 

 which, being characteristic of the mollusean fauna of the Mississippi 

 drainage system, have come to be generally known as " North Ameri- 

 can types of Unio." So unmistakable is their relationship that no rea- 

 sonable doubt can be entertained that the fossil, represent the living 

 forms ancestrally. 



In only one particular, if we except the peculiar plication of U. 

 belliplicatus, do any of the fossil species of Unio of Laramie or Tertiary 

 age assume a characteristic which is either not present or not clearly 

 recognizable in any living species among North American Uniones. 

 This excepted characteristic consists in the extreme shortening of the 

 shell in front of the beaks in certain of the species which have also a 

 considerable transverse elongation and an approximately oval outline; 

 that is, instead of having the beaks situated near, or only a little in 

 advance of, the mid-length of the dorsal border, as they are in all the 

 living oval and some of the shorter forms, the beaks in the case of the 

 fossil forms in question are placed very near to the front. Short forms 

 of Unio, of living as well as fossil species, have their beaks placed thus 

 far forward, but attention is called to the fact that it is only in the fossil 

 species that this peculiarity has been observed in connection with such 

 shells as are much elongated transversely. A living species, which per- 

 haps more nearly than any other approaches in this respect the fossil 

 species referred to, is U. clavus Lamarck, which is a common shell in 

 the Ohio Eiver and its tributaries. But this species is, in reality, 

 only one of the short subtriangular forms, which is a little more than 

 usually elongate. 



The Unio subspatulatus of Meek & Hayden,* from the Judith Eiver 

 beds, which is illustrated on Plate 14, may be taken as an example of 

 an elongate shell with a shortened front, such as has been referred to. 

 In this case, however, there is an unusual narrowing of the shell pos- 

 teriorly. 



Associated with U. subspatulatus, and closely related to it, is another 

 form which was described by the same authors under the name of Unio 

 dancej and which is represented on Plate 17. This species has also 

 been somewhat doubtfully identified in the Laramie strata of Southern 

 Wyoming, further mention of which fact is made on a following page 



Two other species have been obtained from the Judith Eiver beds, 

 namely, Unio deiveyanus Meek & Hayden, | and U. cryptorliynchus 

 White,§ both of which possess the shortened front in connection with 

 a transversely oval outline, which features have already been discussed. 



* U. S. Geol. Sur. Terr., vol . ix, p. 518, pi. 41, fig. 1. 



t U. S. Geol. Sur. Terr., vol. ix, p. 517, pi. 41, fig. 3. 



tU. S. Geol. Sur. Terr., vol. ix, p. 519, pi. 41, fig. 2. 



$ An. Eep. U. S. Geol. Sur. Terr., for 1878, Part I, p. 68, pi. 24, fig. 1. 



