NATURAL SCIENCES OF PHILADELPHIA. 57 



Mr. Lea read portions of a letter from Dr. Lewis, of Mohawk, N. Y., 

 in relation to the habits and growth of Unionidfe. 



In the letter read, Dr. Lewis observes that there are excellent reasons for 

 believing that the dark lines upon the exterior of the Uniones, usually supposed 

 to in irk the annual stages of growth, are really year marks, and in proof ad- 

 duces his observation upon shells found in the Erie canal near Mohawk. This 

 canal, from the 1st of April until the close of navigation, has a depth of from 

 5 to 7 feet; when closed by ice, this level is reduced so that the mud at the bot- 

 tom is frozen to the depth of from 4 to 10 inches. In early spring, the water is 

 drawn down for repairs, and the mud exposed. During summer, the canal is 

 richly supplied with materials for the growth both of the shell and soft-parts of 

 the Molluscs. Thus in summer these Molluscs grow rapidly, whilst in winter 

 there must be a period of repose, during which developement is partially sus- 

 pended, partly because there is less food, but more certainly because the 

 Uniones burrow deeply in the mud to be beyond the reach of frost. "That 

 they do burrow, I know," says Dr. Lewis, by the following facts. In one por- 

 tion of the canal, several years ago, I found a considerable depth of frozen 

 mud enclosing Uniones, Margaritanae and Anodontse all dead. After this mud 

 was softened by the sun, great numbers of those Molluscs made their way to 

 the surface from below. Again, in certain portions of the canal, where the 

 bottom was being excavated to the depth of 12 to 18 inches, early in spring, 

 last year, Uniones were turned up from the lowest depths, while only very 

 few had yet made their appearance at the surface. 



A certain portion of the canal, which some 7 or 8 years ago had been com- 

 pleted for about 8 or 9 years, then produced shells in which none of the speci- 

 mens I could find there presented a greater number of year marks than the 

 period would lead us to expect, if those marks be really year marks. I 

 distinctly recollect having counted the year marks of numerous specimens at 

 the time I speak of, with the fact before me that the shells could not have an 

 age greater than 8 or 9 years " 



In regard to the age at which various species of Naiades become capable of re- 

 producing their species, Dr. Lewis " has noticed that no ova are discoverable 

 in some species until after they have attained two, three or four year marks." 



He thinks " Anodontae mature earlier than Margaritanae, while these latter 

 mature earlier than Uniones proper." 



In reference to the study of the developement of the Unio from its embryonic 

 form, he makes the following suggestions : " I would isolate species from 

 each other in shallow cisterns of considerable area, permitting a small current 

 of water to enter the cisterns, so as not to agitate the stratum of mud at the 

 bottom, and provide a strainer of cloth at the point where the waste water is 

 permitted to escape, so as to retain on its inclined surface whatever minute 

 forms might be floated there. Many germs, no doubt, would thus be easily 

 accessible, without any confusion of species, and by making daily examinations 

 of the deposits on the strainer, the developement of species could be studied 

 up to that point when the true character of the Mollusc begins to be apparent. " 



His efforts to transplant the Unio radiatus froui the lakes to the Erie 

 canal have been unsuccessful, but in the course of them, he observed "that 

 there is not a uniformity either in form or color in different individuals. The 

 appendages of the syphons, as well as I am able to state from memory, do not 

 seem to present the same appearances in detail, in all individuals. I think, 

 however, that the variations in appearance may be ascribed to some habit of 

 the animal or may relate to its momentary condition. Sex may also have 

 something to do with it." 



In a postscript he states that he is inclined to think that the marks observable 

 on Paludinse are year marks. Judging by these and the examination of the soft- 

 parts, Paludinse, he says, begin to bear young about the third or fourth year. 

 When they begin to bear young, two, three, four or five young are the first pro- 



1861.] 



