SILURIAN BRACHIOPODA 55 



Plications. Of the five and six plications visible upon the youngest 



member of the series, three or four appear to exist on that portion of the 

 shell included within the earliest growth-line, i. e., presumptively, the initial 

 shell, and they increase by intercalation until, in the adult, the average num- 

 ber is about sixty for each valve. Concentric lines of growth follow each 

 other with unusual rapidity, particularly in early life 



Summary. Atrypa reticularis, in the development of its beak, fora- 

 men, and deltidial plates, is in essential harmony with the other uniforami- 

 nate shells here discussed The reversal of the fold and sinus is an mterest- 

 ing 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, we judge 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. 



Retzia evax. Hall, 1863 



Plate V, Figs, i-g 



, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. i6o. 1879. 



, Hall. Eleventh Ann. Rept. 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 i 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 Retzia sobrina 

 Rhynchonella indianensis, and Rhynchonella ivhitii, is the sinus which exists 

 on both ventral and dorsal valves; and of much accessory value, the com- 

 paratively slight variation in the number of the plications on the lateral 

 portions of the valves. These features will be presently adverted to more 

 at length. 



