THE NAUTILUS. 113 



deep fossa or excavation below C3, between it and the plate 

 edge, which just at that place is sometimes even raised over 

 the general level, tooth-like, and it may be mentioned that 

 such a form of variabtte has been described as mirabile by 

 Clessin. That fossa is left unfilled for the accommodation of 

 C2, and a similar one is in the left valve, between C2 and C4, 

 for the reception of the posterior part of C3. 



A large number of species are in a broad sense intermediate 

 between these extremes, not counting dubium and amnicum, 

 which have been mentioned above. In many, the hinge-plate 

 is moderately broad with the edge in its whole length some- 

 what projecting, the cardinals are moderately curved, in 

 many partially "buried", with a more or less marked groove 

 below, and with C3i and C3o generally distinct and more or 

 less divergent. Some of these species are: P. noveboracense, 

 abditum, politum etc. 



With the growth in length of the valves, the LAMINA grow 

 distad, but they not only lengthen but move, change their 

 positions, somewhat as the adductor muscles and their inser- 

 tions do; that is, the laminae of the young mussel have dis- 

 appeared in the adolescent and adult, inclosed first in the 

 growing hinge-plate and later in the proximal part of the 

 thicker laminae. With them proceed also the cusps, the fossae 

 of the right valve and the rugose areas described above. As 

 growth in many mussels is not continuous but interrupted 

 hy rest periods, marked by lines on the outer surface, so it is 

 to some extent with the laminae, and they often can be dis- 

 tinctly seen in al and all of the larger Sphasria. 



At the same time the lamina? grow medianward, as the 

 cardinals do, and show a more or less distinct curving up- 

 ward. Also they gain in thickness, though moderately so in 

 Spharium and little in most species of Musculium. With 

 both of these, the hinges do not change very much in shape 

 during growth, after the nepionic stages, and also the essen- 

 tial features of the mature mussels are rather the same in the 

 several species of each genus. (The shape of the lamina? of 

 a Sphasrium is shown in fig. 3, with the "history" of its 

 growth). 



