PHYLOGENY OF THE PELECYPODA. 315 



regular growing, unworn, young specimen. There ai-e often well defined stages indi- 

 cated as arrests of growth in the early growth of the spat ; but I must retract the state- 

 ment that there ai"e typically five and that they are always more or less clearly marked in 

 regular, imworn, young specimens. Frequently no indication of later spat stages are 

 present, and they cannot be considered as constant. To remodel my conclusions and 

 statement: The spat stage in Ostrea virginiana, as a whole, is frequently marked by 

 stages of growth. Stages are characteristic of the earliest spat growth and as many as 

 five have been found, but often no more than two or three are indicated. To study these 

 stages specimens should be sought growing in protected places, as the wear of sand 

 and waves destroys their demarcation rapidly. Some spat found in the inside whorls of 

 dead Sycotopus shells and others, secured on glass in di-ain pipes and on inverted flower 

 pots, retained the markings clearly. 



The spat growth begins along the ventral margin of the prodissoconch valves 

 PI. XXIV, figs. 18-21 and PI. xxv, figs. 3-4, and throughout life the most rapid growth 

 is in this direction. The growth rapidly pushes up the sides of the valves, and between 

 the hinge areas of the latter, PI. xxv, fig. 3, lifting them apart and separating them by 

 interstitial growth. A little to the right of the umbo of the lower, loft prodissoconch 

 valve, as seen from above, in the initial stages of spat growth, is a notch which indicates 

 the beginning of the cartilage pit of the lower valve. This is seen clearly in PI. xxiv, fig. 19. 

 It is necessary to describe the left and right valves separately. The upper, right valve, 

 PI. XXIV, fig. 20, is continuous with the prodissoconch valve and follows the cui've of that 

 stage though in a lessening degree. It spreads out laterally on the hinge line in a de- 

 scending curve, and on the margin overhangs the lower valve by a considerable amount, 

 PI. XXIV, fig. 21, as it continues to do while the shell is flatly related to the object of 

 fixation. The same oveilianging of the upper valve may be seen in young Pcctens, 

 PI. XXVIII, and Anomias, PI. xxix. The lower left valve of the young oyster, PI. xxrv, 

 fig. 21, starts as does the upper, following the curve of the early valve. When this curve 

 has been followed for an extremely brief period, the valve suddenl}^ flattens and becomes 

 closely i-elated to the surface of attachment. The result is, that a slight groove runs 

 around the border of the spat valve, beyond which it is abruptly continuous Avith the flat 

 later growth as mentioned by Ryder. I have, however, to note a peculiar and significant 

 exception. Sometimes, instead of becoming flat and closely related to the object of at- 

 tachment, the spat valve is continuously curved throughout its early growth, and not 

 at all flattened. It then i-esembles the upper valve in shape. This cui'ved shape is 

 held, as far as observed, only on rough objects of attachment. Seeking for such mate- 

 rial on rough stones and shells, one will always find some valves more or less curved in 

 their earliest stages. This is interesting, as showing the modifying influences of envi- 

 ronment on a very young stage, where heredity might be supposed to fashion the form 

 with overpowering force. Further notes on this character are given when discussing 

 Exogyra, section vii. 



When the spat shell has attained the height of 0.8 mm. to 1.2 mm., frequently a well 

 defined stage or pei'iod of growth is marked by an interruption in the lines of gi-owth as 

 shown in PI. xxv, figs. 1-2. This indicates what is considered as tlie ch)se of the fii-st 

 nepionic stage. In the shell growtli succeeding the first nepionic stage the upper valve, 



