SILURIAN BRACHIOPODA 25 



having a length of 2.25 mm. The dorsal valve shows a foramen in the 

 cardinal area under the beak, margined by a slight thickening of the shell. 

 The ventral valve preserves a small perforate pedicle-tube at the apex, 

 extending about two-thirds of the distance down to the hinge, below which 

 is a triangular deltidial opening of the same width as the dorsal foramen. 



A specimen 4 mm. in length (plate 2, fig. 19), shows a more advanced 

 development of the same parts. The dorsal callosity has nearly filled the 

 sinus under the beak and has a narrow groove in the center. The fissure 

 of the ventral valve has increased considerably in size and relative height, 

 showing narrow marginal plates, or defined areas which may be deltidial 

 plates. The pedicle-tube is still perforate, but has not increased in size 

 beyond the initial stage. 



From this point to maturity, the hinge increases in width, the dorsal 

 callosity grows rapidly and nearly fills the fissure of the opposite valve. 

 The pedicle-tube is obscured and the perforation obsolete. The deltidial 

 plates, or lateral areas, are clearly defined, and have the form of narrow 

 scalene triangles. 



No important variations have been noticed among the mature speci- 

 mens. Occasionally an individual diverges from the normal form by having 

 mucronate cardinal angles, or a senile specimen shows strong imbricating 

 varices of growth, but, as a whole, the form and surface ornaments in this 

 species are very uniform. 



Strophonella striata, Hall, 1843 



Plate III, Figs. i-8 



Strophodonta striata. Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 152, pi. 23, 



figs. 1-6. 1879. 

 , Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 290, pi. 23, figs. 1-6. 1882. 



The present form is one of the most delicate and fragile species of 

 brachiopoda at Waldron. Individuals are not of rare occurrence, but the 

 majority of them are more or less broken. The upper portion of the shell, 

 4 



