INYEETEBKATE 1'AL/EONTOLOGY. 515 



14. metaptera, Raf. (zr Proptera, Rat'., and Lymnadia and Megadonn/s, 

 Swainson). 

 Shell with hinge-margin elevated and winged, the valves connate, 

 and the surface plicate or smooth. — (TJnio alatus, Say.) 



To the foregoing may be added a subgenus proposed by me under the 

 name Loxopleurus, for the reception of a very peculiar species, from near I he 

 junction of the Cretaceous and Lower Tertiary of Wyoming Territory.* It 

 is an elonijate-subovate, somewhat arcuate, shell, without wings, and orna- 

 mented with two sets of very regular, well-defined costfE, those of one of 

 which start from the margin just in front of the beaks, and radiate obliquely 

 backward and downward ; while those of the other set start from the dorsal 

 margin behind the beaks, and extend downward, in a direction that would 

 ' cause them to intersect those of the other series at rather acute angles along 

 the posterior umbonal slopes, somewhat as in Goniomya. 



Species have been referred to the genus (Jnio,exen from the Coal-Meas- 

 ures of Europe; but it is now believed that they all belong to the genus 

 Carbonocola and other distinct genera. Nor have we any satisfactory evi- 

 dence yet of the. existence of true Uniones during the Permian epoch. I 

 have described a few species, however, from a bed in New Mexico, in which 

 Professor Cope found vertebrate remains, that lead him to refer the rock to 

 the Triassic period. Another species that appears to belong to this genus 

 was some years back described by the writer, in connection with Dr. Hayden, 

 from Jurassic beds near the Black Hills, Dakota. The genus is well known 

 to be represented in the Wealden deposits of Europe, and occurs in beds in 

 the Far West, belonging to the Upper Cretaceous. It is also abundantly 

 represented at various horizons in the fresh-water Tertiary deposits of the 

 Western Territories, as well as in various other parts of the world. 



At the present time, this genus is widely extended, in Europe, Asia, 

 Africa, New Holland, and America. The streams and lakes of the United 

 States, however, seem to be, as it were, the home of the Uniones; the num- 

 ber of species here exceeding all of those known from all other parts of 

 the world, while they assume a greater diversity of forms and other char- 

 acters than have been observed elsewhere. 



* The type of this section ( U. beWplicatus) is figured in the Palreontological Report of Mr. King's 

 Geological Snrvey of the Fortieth Parallel. 



