SILURIAN BRACHIOPODA 63 



The spiral brachial supports each make about five revolutions, 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 result- 

 ant of a very completely represented series of embryonic stages. The spe- 

 cies is also subject to obese growth, resulting from two sources ; (a) general 

 internal thickening of the shell, (<$) 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 5, figs. 10, io). The first stage of growth 



represented in our series measures 2 mm. in length by 1.6 mm. in width. 

 The valves are subequally convex, somewhat depressed anteriorly. Outline 

 broadly ovate. 



Ventral valve with beak high, erect, and subacute ; cardinal slopes 

 broad, steep, and slightly excavate ; pedicle-aperture subtriangular, rounded 

 at the apical angle, and also slightly, at the base, by the already developed 

 deltidial plates. 



Dorsal valve subcircular, beak full, rounded, sides slightly appressed, 

 apex concealed. Surface of the dorsal valve marked by two thread-like pli- 

 cations which take their origin medially, just below the umbonal region ; 

 thenceforward they rapidly diverge, forming the embryonal sinus, which is, 

 however, soon filled by two small plications. The latera each bear one 

 plication, of earlier age than the sinal, and later than the primary plica- 

 tions. On the ventral valve, the plications 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 

 appearance 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. 



