364 STUDIES IN EVOLUTION 



a beak incurved to such a degree as almost to obscure the 

 foramen. On approaching maturity the deltidial plates ap- 

 pear to become anchylosed along their exterior edges, with 

 the shell itself, the line of union being marked with a low 

 ridge, and seem never to be displaced by any distortion of the 

 shell, as so often occurs in Atrypina disparilis. 



Sinus. As already noticed, in the elementary shell the 

 sinus begins as a low, smooth depression, equally strong on 

 both valves, and extending almost to the beak. It gradually 

 becomes filled by the radiating plications, which appear first 

 at the sides, and increase toward the middle, never becom- 

 ing, normally, more than six. None of these plications 

 reach the apex of the shell. 



Plications. On the latera of the shell these seem to appear 

 simultaneously, as shown in figure 1, where three on each 

 side make their appearance at the same stage of growth. 

 This number is subsequently increased to six or eight on 

 each side in mature forms, sometimes becoming greater in 

 old age. 



internal Apparatus. The brachial supports in this species 

 consist of spirals, coiled in the transverse axis of the shell, 

 with their bases facing each other. In the mature individual 

 the number of coils is from eight to ten. The spirals are 

 connected by an angular loop, the branches of which take 

 their origin on the dorsal limb 'of the basal coils, and are 

 directed ventrally and backward beyond the axis of the in- 

 terior cavity, forming at their junction not a simple angle, 

 but a miniature saddle, from the posterior extremity of which 

 extends straight backward a little spiniform process. The 

 number of coils in these spires varies with the age of the 

 shell. In preparing a series to show the development of 

 these structures, it appears that the shelly ribbons composing 

 the spirals not only make fewer coils in early life, but that 

 these are of exceeding tenuity in the primary stages of de- 

 velopment. The accompanying figures show the extremes of 

 development noticed in these respects, figures 126 and 127 

 representing the character of the supports in the mature 



