NO. 1205. SYNOPSIS OF THE NAIADES SIMPSON. 511 



is the highest type of a branchial uterus. Lampsilis is a step farther 

 on, with oval to elongated shells, generally highly colored, with, as a 

 rule, a decided post-basal swelling of that of the female, without (in 

 most cases) a strong post ridge, and the beak sculpture consists of 

 rather numerous, delicate bars, arranged in two distinct (anterior and 

 posterior) loops. 



Throughout the entire Mississippi Valley, a portion of the Atlantic 

 drainage, in most of the streams flowing into the Gulf of Mexico, in east- 

 ern Mexico, and Central America there is found^au extensive group of 

 Uniones for which I have used the name Proptera of Rafinesque. Nearly 

 all the shells are more or less dor sally winged, the beak sculpture is 

 feeble, the epidermis is dull, the teeth are often imperfect, and they have 

 a row of dorsal scars running obliquely downward and forward. The 

 nacre generally is some shade of purple, and though the female shell 

 may show a decided post inflation, it may be wanting. The ovisacs 

 are usually fine and numerous, and are placed in the posterior ends of 

 the outer gills. I have given this group subgeneric rank under Lamp- 

 sills. Now, it would seem probable that North America was the field 

 in which these remarkable developments in the character of the TJuioii- 

 ida- had taken place, for I know of no fossil species elsewhere showing 

 any of these higher characters. If, as I have elsewhere suggested, 

 there is a relationship between Proptera and the Cristarias, Hyriopsis, 

 Pilsbryoconclia, Chamlerlainia, and Pseudospatha of the Old World, 

 then I should think it likely they, or their progenitors, had migrated 

 thence from North America some time during the early or middle ter- 

 tiaries. It is possible that the Cristarias, Hyriopsis. and the like, or 

 closely related but now extinct groups, may have originated in ori- 

 ental waters, and that the Propteras, and the typical LampsiUs, may be 

 their offspring. 



Truncilla marks the highest development of Naiad life, and may 

 be taken to be the latest. Its shells are smooth and generally highly 

 painted, the beak sculpture is fine and doubly looped, the hinge teeth 

 are well developed. The post-basal area is very distinctly marked out 

 and developed in the female. In most cases the shell of this part in 

 the female is thin, of a different texture from the rest, often radially 

 ridged, and decidedly toothed on its edge. There is usually a surpris- 

 ing difference in the shape of the shells of male and female aside from 

 this, si) much so that one would never suppose that they belonged to 

 the same species, unless he traced the growth back to the young shells, 

 which are quite alike in the different sexes. The marsupium is like a 

 large kidney, very full, and totally different in appearance when 

 gravid from the rest of the gill. It appears to be protected by a great 

 flap which grows out from the mantle covering it, which is here double. 

 The genus exhibits a great variety of form, so much so that several 

 good subgeneric groups seem well marked out, and we must believe it 

 has been in existence for some time. 



Much of the foregoing may be mere conjecture; much is undoubt- 

 edly founded on fact. I believe that the living forms of the Uuiouidse 

 show a gradual development from the simplest, lowest, and earliest 



