368 STUDIES IN EVOLUTION 



The spiral brachial supports each make about five revolu- 

 tions, which come to an apex near the lateral margins. 



Variations from the Normal Mature Form. — An elongate 

 form with an unusually high and straight beak is not of rare 

 occurrence, and is the resultant of a very completely repre- 

 sented series of embryonic stages. The species is also sub- 

 ject to obese growth, resulting from two sources : (a) general 

 internal thickening of the shell, (b) marginal thickening. 

 Both are the result of post-adolescent or senile growth, the 

 former producing a round, full, plethoric shell, the latter 

 giving the shell a truncate appearance. 



Incipient Form (Plate XIX, figures 10, 10 a). — The first 

 stage of growth represented in this series measures 2 mm. in 

 length by 1.6 mm. in width. The valves are sub-equally 

 convex, somewhat depressed anteriorly. Outline broadly 

 ovate. 



Ventral valve with beak high, erect, and sub-acute; car- 

 dinal slopes broad, steep, and slightly excavate; pedicle- 

 aperture sub-triangular, rounded at the apical angle, and also 

 slightljf at the base, by the already developed deltidial plates. 



Dorsal valve sub-circular, beak full, rounded, sides slightly 

 appressed, apex concealed. Surface of the dorsal valve 

 marked by two thread-like plications which take their origin 

 medially, just below the umbonal region ; thenceforward they 

 rapidly diverge, forming the embryonal sinus, which is, how- 

 ever, soon filled by two small plications. The latera each 

 bear one plication, of earlier age than the sinal, and later 

 than the primary plications. On the ventral valve the plica- 

 tions number the same, but the embryonal fold or dorsum is 

 more strongly marked than the dorsal sinus. On both valves 

 indications of the mature fold and sinus begin with the appear- 

 ance of the sinal plications. 



Developmental Variations. 

 General Form and Outline. — There is a gradual increase in 

 convexity and diameter with each successive growth-stage, 

 until maturity is reached. The elongate variation from the 



