SILURIAN BRACHIOPODA 59 



oval, and a circular outline. The circular curve of its upper extremity 

 is caused by a slight encroachment upon the beak, and this in mature 

 age is so considerable that the primary or incipient shell is undoubtedly 

 wholly absorbed. The plane of the foramen remains, except in rare 

 instances, always vertical, although the deltidial plates become slightly 

 bent by the incurving of the beak. A striking exception to this rule is 

 represented on plate 5, figure 9, where an individual which has reached 

 early maturity shows the senile feature of a beak incurved to such a degree 

 as to almost obscure the foramen. On approaching. maturity, the deltid- 

 ial plates appear to become anchylosed along their exterior edges, with 

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

 have never seen them displaced by any distortion of the shell, as so often 

 occurs in Coelospira disparilis. 



Sinus. As already noticed, the sinus, in the elementary shell, 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 

 becoming, 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 simul- 



taneously, as shown in figure i, 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 ooils 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 ven- 

 trally and backward to beyond the axis of the interior cavity, forming at 



