AET. 11 ANATOMY OF FRESH-WATER MUSSELS REARDON" 7 



ANATOMY OF LAMPSILIS NASUTUS SAY 

 Plate 2, figs. 1 to 10 



DESCRIPTION OF THE SHELL 



Plate 2, figs. 1, 2, 4 



The shell is not shiny and ranges in color from dark brown to 

 olive green; it is thin to moderately solid, elongated, and has a dis- 

 tinct posterior ridge, which generally is curved down the middle. 

 The anterior ventral margin curves broadly to the base which is full 

 behind the center, attenuating to a long point halfway up the height 

 of the shell. * 



The highly characteristic beaks are low and sculptured with fine 

 close ridges, looped in front and following the paralleling longitudinal 

 axis of the shell behind. (PI. 2, fig. 5.) The exterior surface shows 

 irregular growth lines, with sometimes vertical bars just back of the 

 center, representing the growth of the shell over the gills while dis- 

 tended with fry. Young shells are finely rayed, while in the adult 

 shell the rays are less distinct or entirely invisible. 



The interior of the shell is iridescent and lustrous and may be 

 bluish-white, lurid, flesh-tinted, or purplish. The left valve has two 

 pseudo cardinal teeth and one lateral hinge tooth, which is nearly 

 straight and very delicate. The right valve has one pseudocardinal 

 tooth with a faint one above it and two lateral hinge teeth. The 

 muscle scars are shallow, the anterior adductor scar being the more 

 prominent. It is situated dorsally near the anterior margin of the 

 shell and is oval shaped and very clear cut. The posterior adductor 

 scar is a large faint oval impression just below the termination of 

 the hinge teeth. Dorsal to it is the small circular impression of the 

 posterior retractor muscle. On the posterior ventral margin of the 

 anterior adductor muscle scar are two small irregular impressions, one 

 dorsal. They are the anterior retractor and the protractor pedis 

 muscle scars, respectively. Beginning at the most ventral point of 

 the anterior adductor muscle scar, there is a thin faint impression 

 running parallel with the margin of the shell and about one-fourth 

 inch distant from it and ending at the base of the posterior adductor 

 muscle. It is the pallial line impression and marks the insertion of 

 the small pallial line muscles. There are two small pitted scars in 

 the beak cavities which mark the insertion of the small dorsal 

 muscles. 



MANTLE 



Plate 2, fig. 3 



From the posterior adductor muscle the margins of the mantle are 

 separated for the length of the muscle to form the dorsal mantle 

 aperture. The edges reunite for a short distance and then separate 

 again to form the exhalent siphon, the lower portion of which is 



