30 THE NAUTILUS. 



from any Unios ; and this seems to be the character on which the 

 writer would separate Margaritmta generically from Unto. 



In the former species these little muscle scars or points of attach- 

 ment of the mantle are sometimes a set of round, deep punctures in 

 the nacre, but more often they consist of slightly indented dashes, 

 which radiate from the umbonal cavity. They vary in number from 

 a very few to 50 or more, and are often entirely wanting. In some 

 examples these scars are more or less aggregated into a sort of lon- 

 gitudinal row along the middle of the disk, looking like a strongly 

 developed pallial line. 



In Margaritanamonodonta they appear usually as deep punctures, 

 and vary from many to none and the same thing is true of Unio hem- 

 In ti. I have not found them in U. decumbens or U. lui>*esis. 



In 1830 Isaac Lea described Unio trapezoides in the Transactions 

 of the American Philosophical Society, Volume IV, page 69, and 

 called attention to the fact that this species possessed a strongly 

 developed muscle scar near the center of the disk, which he then 

 named the ventral cicatrix. It is present (sometimes double) and 

 well developed in most specimens, feeble in others, or it may be found 

 in one valve and wanting in the other, or absent altogether. The 

 same is true of most of the species of the plicate group of Unios, 

 which are all nearly related; N. multiplicatus, inululiitu*, perji/i<-n- 

 tus, etc., but I have never found these scars in the nearly allied U. 

 xlniitiann-8 Lea, of Georgia, which is so close to U. trapezoides that 

 Call has placed it in the synonymy of that species. 1 In V. tm/n-- 

 zoides there may be one or two anterior pedal scars and they are 

 often widely separated. 



A wonderful degree of variation is also found in the number and 

 position of the dorsal scars of many species of Unios, and in the 

 degree of development of the scars in the pallial line. In Mr. B- 

 H. AV right's new Unio, U. bursa past or is, from Tennesseee, the 

 pallial line is generally composed of deep, strongly marked scars, 

 to which the mantle is attached ; in Unio ventricosus it is often so 

 faint as to be scarcely discernable. I know of no character more 

 variable and wholly unreliable as a means of classification in the 

 Uriionidce than that of the muscle scars and my studies lead me to 

 believe that it is seldom a mark of even specific value. 



'Tr. Acad. Sci. St. Louis, VII, Xo. 1. p. 54. 



