360 STUDIES IN EVOLUTION 



reversal of the fold and sinus is an interesting but not 

 unique feature, and by the time it has been completely 

 effected many of the characters of maturity have been 

 assumed. From the degree of exposure of the foramen, 

 it may be judged that the animal remained attached by its 

 pedicle up to adult growth, but with full maturity and the 

 approach of senility the pedicle must have become atrophied 

 and the animal set free. 



Homceospira evax Hall, 1863. 

 (PLATE XIX, figures 1-9.) 



Retzia evax Hall. Twenty-eighth Ann. Kept. N. Y. State Mus. Nat. 



Hist, p. 160, 1879. 

 Hall. Eleventh Ann. Kept. State Geol. Indiana, p. 302, 1882. 



In this species the superficial features have been found of 

 much more permanent character than is usual in the plicate 

 Brachiopoda from this horizon. Not far from three thousand 

 individuals have been examined, and these show a variation 

 in size from a length of 1 mm. and a width of .8 mm. to a 

 length and width of 25 mm. Throughout the younger stages 

 in this series of variations the feature of primary importance 

 in distinguishing the embryo of this from those of other 

 species, notably Homoeospira sobrina, Camarotcechia indianen- 

 sis, and Camarotcechia Whitii, is the sinus which exists on 

 both ventral and dorsal valves ; and of much accessory value, 

 the comparatively slight variation in the number of the plica- 

 tions on the lateral portions of the valves. These features 

 will be presently adverted to more at length. 



Specific Characters. 



Mature Form (Plate XIX, figures 2, 2 a). Shell ovate, 

 generally longer than wide, both valves almost evenly con- 

 vex, and of about the same depth. Anterior margin gener- 

 ally slightly emarginate, on account of the median sinus 

 which exists on both valves. In rare instances a low median 

 fold is developed near the margin of the dorsal valve. 



