316 ROBERT TRA.CY JACKSON ON THE 



fig. 1, is ill form a continuation of the first stage. Tlie lower valve spreads out over the 

 object of attachment by wing-like extensions of its anterior margins, and makes a shoulder 

 on its anterior border by its increased growth, where it comes in contact with the outer 

 limit of the first stage, fig. 2, and similar shoulders mark the periods of growth of later 

 stages in the lower valve, PI. xxv, fig. 6. The lower valve may remain curved, as in the 

 first nepionic stage, for a limited extent. When the spat has attained the height of from 

 about 1.4 ram. to 2.4 mm., another stage is frequently marked by an interruption in the shell 

 growth as shown in PI. xxv, figs. 1-2, which illustrate the first and second stages of the 

 nepionic or spat growth. After the second stage, as desci-iljed, other stages maybe found 

 in the spat growth. They are mai-ked as arrests in the regularity of shell growth and in 

 the lower valve shoulders are formed by their contact Avith earlier stages as shown on 

 the hinge border in PI. xxv, fig. 2. In all, five stages have been observed; but the num- 

 ber and the later stages cannot be considered as charactei'istic as the first two stages are. 



After the close of the second spat stage the shell is still even and rounded in outline, 

 as the convolutions and distortions connnon in oysters do not often begin till growth has 

 considerably advanced. The lower valve spreads out over the object of attachment, and 

 leaf-like layers of shell deposition may be seen spreading beyond the limits of the hinge 

 line, as growth continues, as shown in PI. xxiv, fig. 19, and PI. xxv, fig. 5. The upper 

 valve in regular growing specimens frequently departs from the form which is ob- 

 served in the earlier growth, PI. xxiv, fig. 20, and spreading out laterally becomes more 

 wing-like on the mai-gin than in any earlier stages, PI. xxv, figs. 5 and 7. These wing- 

 like productions of the spat growth were pointed out and figured by Ryder (60). In 

 describing such stages he says: they are " .... handsomely rounded in shape, not 

 unlike a scallop in form, which they simulate still more by the lobes which they develop 

 on the shell on either side of the hinge." This scallop-like form, shown in PI. xxv, 

 fig. 7, is qiiite likely due to the irritating action of a foreign body on the extended mantle 

 border of the left valve, or it may be due to the opportunity to make more secure the 

 fixation by dejiosition of lime in this direction. In spat growth the free ends of the 

 valves often change the direction of their gi-owth to the left or right of the median line. 

 By this change the umbos may be revolved so that they often point in the opposite di- 

 ]-ection fi-om that which they woidd have otherwise assumed, as noted by Ryder (65). 

 A twist in the direction of growth as described is characteristic of the fossil genera, Ex- 

 ogyra and Gryph^ea, in which the variation is always to the left of the median line viewed 

 from above. The young oyster when it twists to the left resembles very much a young 

 Exogyra; but when the twist occurs in the opposite direction, or to the right, as fre- 

 quently occurs, this close comparison cannot be made. The right valve of the spat has 

 a sinuosity, si, PI. xxv, fig. 5, which is related to the excurrent flow of eftete water as 

 described, p. 307. 



Throughout the period of spat growth the young oyster shell has an approach to the 

 rotundity characteristic of the early spat stages. The attached valve is flat and the fi-ee 

 convex, but vyhen the oyster projects beyond the object of fixation this relative form of 

 the valves is changed and the condition characteristic of the adult, PI. xxv, fig. 8, is 

 brought about. The attached valve becomes deeply concave or spoon-shaped and tlie 

 free valve conversely Ijecomes flat, as pointed out l)y Brooks and Ryder. At the close 



