53 



The valves of larvae are connected by a ligament. This ligament may be 

 external or internal to the hinge line (see Figure 18). The ligament is termed 

 anterior, median, or posterior according to its position relative to the midline 

 of the hinge. In identification of larvae the following parameters and characters 

 are taken into consideration. 



1. Size parameters of larva : Length, height and thickness of shell (Figure 

 34). Length of shell is the maximum distance between the anterior and pos- 

 terior margins parallel to the hinge line. Height of shell is the distance from 

 the apex of the umbo to the ventral margin perpendicular to the hinge line. The 

 thickness, which characterizes the degree of convexity of valves, is the dis- 

 tance between the most convex areas of the valves. The length of the anterior 

 and posterior ends of the shell may be used as an auxiliary parameter; it is the 

 distance from the anterior or posterior margins of the shell to the line of height. 

 Many researchers (Rees, 1950; Zakhvatkina, 1959; Loosanoff et al., 1966; 

 Gal'perina, 1969; Chanley, 1968; Schweinitz and Lutz, 1976; and others) use 

 relative size parameters (ratio of shell height to length and height to thickness) 

 as identification characters. 



2. Shape of larva : The shape of a larva is determined by the following 

 characteristics : (a) length and mutual disposition of shoulders (these may be 

 equal or unequal in length and slope to the umbo at a definite angle or parallel 

 to each other); (b) length and shape of anterior, posterior, and ventral margins 

 of the shell (these may be straight or curved, sharp or blunt, and also slope to 

 one or the other end); and (c) shape and size of the umbones (these may be 

 knob-shaped, slanted, round, flat, or obtuse) (Figure 35). 



Figure 35: Shape of umbones of larvae, 

 knob-shaped; B — slanted; C — irregularly rounded; D — obtuse; E — angular. 



