330 STUDIES IN EVOLUTION 



perforation obsolete. The marginal plates, or lateral areas, 

 are clearly defined, and have the form of narrow scalene 

 triangles. 



No important variations have been noticed among the 

 mature specimens. 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 XVII, figures 1-8.) 



Strophodonta striata Hall. Twenty-eighth Ann. Kept. N. Y. State Mus. 

 Nat. Hist., p. 152, pi. 23, figs. 1-6, 1879. 



Hall. Eleventh Ann. Kept. 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, or that along the 

 hinge, being thicker and stronger than the remainder, is more 

 often preserved, and the series is only complete in the repre- 

 sentation of this portion, although there are several small 

 specimens which are sufficiently entire to show the early 

 form of the shell. 



As in the other species which in their mature proportions 

 depart from the type of structure in the group, the incipient 

 shell is found to revert to the primitive form. The full-grown 

 examples of this species are concavo-convex, the concave 

 valve being the ventral ; while in the young the ventral valve 

 is the more convex. This change in the relative convexity 

 of the valve does not begin until the individuals are about 

 half grown, and is produced by the gradual deflection of the 

 margin with the increase in the size of the shell. 



The development of the features of the hinge is very char- 

 acteristic, and, as in the other strophomenoid forms, is of 



