1888.] 545 [Jackson. 



the outlines of the first silphologic stage. The convolutions and 

 distortions common in oysters do not often originate before the 

 close of this stage. The lower valve spreads out over the object 

 of attachment, and the leaf-like layers of deposition are seen as 

 viewed from above, spreading beyond the limits of the hinge line, 

 (see fig. 14, pi. vn). I have not seen an oyster in which this stage 

 remained curved as described in the two previous stages. 



Fourth silphologic stage. — This stage and the fifth are more dif- 

 ficult to characterize than the preceding ones, as so much variation 

 occurs in different specimens. The upper valve in regular grow- 

 ing specimens frequently departs from the angle of the anterior 

 marginal line which is handed down from the first silphologic stage 

 and spreading out laterally becomes more wing-like on the margin 

 than any earlier stages, fig. 14, pi. vi. These wing-like produc- 

 tions of the spat growth were pointed out and figured by Professor 

 Ryder in a description of the spat stage (21, 24). In this stage 

 the shells often change the direction of growth and the umbos may 

 be revolved so that they point in the opposite direction from that 

 which they would otherwise have normally assumed. 1 The lower 

 valve still remains flat and throws out extensions of its margin over 

 the object of attachment as shown in Ryder's figures referred to 

 and my fig. 14, pi. vi. 



Fifth silphologic stage (fig. 7, pi. v). — This may be described as 

 similar to the fourth stage, and it is the last of the spat stages of 

 growth. The end of this stage is very marked and may commonly 

 be seen in adult oysters as the limits of the smooth, rounded con- 

 vex area at the apex of the conch, and is often figured in living 

 and fossil oysters. 



The shell has, up to this time, normally, the rotundity charac- 

 teristic of the earlier spat stages ; but afterwards, with certain ex- 

 ceptions it assumes the true ostrean form. If the oyster continues 

 to grow closely on the object of attachment, the lower valve re- 

 mains flat and the upper curved as before, fig. 7, pi. v, but no 

 more distinct stages are indicated. When the shell projects over 

 the object of attachment the lower valve becomes concave and the 

 upper flat, as in adults, as described by Brooks (4) and Ryder 

 (21). If this occurs before the close of the fifth silphologic stage 

 it does not prevent the demarcation of those stages in both valves. 



1 Professor Eyder (25) notes this change in direction of growth. 



PROCEEDINGS B. S. N. H. VOL. XXIII. 35 JULY, 



