Grier: Morphological Features of Mussel-shells. 149 



Wilson & Clark (59) state that Anodonta grandis is lacustrine by 

 choice, and that in lakes the shell is typically inflated and thinner, 

 while in creeks it becomes it becomes thicker, more compressed, 

 elongated. Later (61) they note that where two closely related 

 forms of Naiades differ essentially in the degree of inflation of the 

 shell, the flatter and less inflated form is found in the upper portions 

 of the river and in its tributaries, while the rounder and more inflated 

 form is confined to the lower stretches of the main stream where there 

 is a weaker current and more mud. They found, however, a notable 

 exception in the case of Syjiiphynota {Lasmigona) costata. They also 

 note that the swiftness of the current, the size of the stream, and the 

 kind of bottom affect other characters of the shell besides its degree 

 of inflation. Further (60) they say: "Below the Cumberland Falls 

 in limestone formations the water contains a considerable percentage 

 of lime. Here the shells are much larger and thicker than above the 

 Falls." Danglade (11) writing with regard to the Illinois River, 

 states that the mussels become smaller in the lower stretches of the 

 stream. He states with regard to Quadrula metanevra that one of the 

 examples before him has ' the markedly pinched posterior dorsal 

 portion' generally found in the lower Wabash. He says of Quadrula 

 undulata that it ' becomes inflated lower down the river.' With regard 

 to Obliquaria rcflexa he says: "In the Peoria lake-region, where the 

 current is slow and the bottom is composed of soft mud, the shell is 

 often very heavy and rounded anteriorly, while posteriorly it is thin 

 and much elongated, which is no doubt the result of accomodations 

 to natural conditions. In lower stretches the shell, although heavy 

 and inflated, is considerably smaller than those in the upper portions 

 of the river." Of Plagiola elegans he remarks: "Some examples from 

 portions of the river having soft and mud bottoms are greatly elongated 

 posteriorly." Utterback (51, 52) notes that in the Osage River 'flat 

 or compressed forms are found at the head-waters where the water is 

 shallower or swifter, and that they become heavier and more swollen 

 further down stream, where the water is deeper and more sluggish.' 

 He also found that the quiet, sluggish streams of northern Missouri 

 tend to produce a heavy, rarely plicated, highly inflated shell {Am- 

 blema) ; on the other hand the swift clear water of the streams of 

 southern Missouri give origin to compressed multiplicated shells. 



Objection to the statement that Naiades become more inflated in 

 the lower reaches of streams is offered by that eminent student of the 



