98 THE NAUTII.LS.. 



toward tiie posterior end. Anterior margin incrassate. Posterior 

 dorsal curve regular and strong. Posterior umbonal slope flattened, 

 and separated by a decided angle from the lateral slope. There are 

 some traces of waviness on this posterior slope. The umbonf s are 

 not very prominent and but very slightly incurved ; the ligament 

 long and heavy. The cardinal teeth, though much weathered, were 

 evidently short and heavy, the lateral teeth long and nearly straight. 

 The anterior adductor cicatrix is large and deep, strongly pitted ; ilie 

 protractor impression triangular. The pallial (cicatrix is very <l(^ep, 

 and crenulate. Only a small [)ortion of the posterior cicatrix re- 

 mains. 



On comparing tlie valve with the Unios in our collection, I am 

 forced to the conclusion that we have here a specimen of Unio crassi- 

 dens Lam. I am further strengthened in this belief on comparing 

 the specitnen with the figures and description of U. ciassidens by 

 Call (a study of the Unionidae of Arkansas, etc. — Trans. Ac. 

 Sciences, St. Louis, Vol. VII, pp. I-60, |)lates I-XXI). Finally I 

 have compared it with two specimens of U. crassidens. one from the 

 Ohio river, the other from southern Michigan, which were kindly 

 sent to me for this purpose by Dr. W, S. Strode, of Lewistown, Illi- 

 nois. It may be that further material will force us to recognize it 

 as a separate variety, but I do not think it probable. 



Now let us consider the present distribution of U. crassidens. 

 According to Call (/. c.) it is abundant in the Cumberland river of 

 Tennessee as well as in other rivers of that State. It occurs 

 abundantl}' also in the Coosa and the Alabama, in the Tombigbee, 

 and southeast to the Chattaboochee (Simpson, Synopsis of the 

 Naiades, Proc. U. S. Nat. Mus., Vol. XXII, pp. 501-1044). It 

 also occurs in the Mississippi and its eastern tributaries as far north 

 as the forty-second parallel ; or, in general, to northern Illinois and 

 southern Michigan. It does not occur within the Basin of the Great 

 Lakes, neither has it ever been found in any stream west of the 

 Mississippi so far as I know. We must look upon it then as essenti- 

 ally a southeastern form, with its center of distribution lying prob- 

 ably somewhere in the rivers of Tennessee. 



We are thus confronted with the problem of its occurrence, in 

 fossil form at Green Bay, in the St. Lawrence Basin. It is because 

 this involves an interesting point in the causes aflfecting present 

 geographical distril>ution, that this note is written. 



