DEVELOPMENT OF SOME SILURIAN BRACIIIOPODA 363 



its size about .5 X .4 mm., this fact in itself is sufficient 

 apology for the writers' not having detected the earliest stage 

 of its development, even if other causes had permitted its 

 preservation. 



Beaks. — In all normally developed individuals less than 5 

 mm. in length the beak of the ventral valve is erect and 

 exsert. At about this stage of growth a tendency to apical 

 incurvature is manifested, which increases up to maturity, 

 when, under normal development, the entire umbo is evenly 

 incurved, concealing the deltidial plates and often much of 

 the foramen. On the dorsal valve the beak is quite obscure 

 in the youngest forms, and in later stages of growth is con- 

 cealed beneath the deltidium or incurved beak of the opposite 

 valve. 



Foramen. — This appears first as a simple triangular open- 

 ing, its apex reaching to, but not truncating the apex of the 

 umbo, and it is retained in this condition until the shell 

 attains a length of at least 3 mm. of normal growth. At 

 this age the deltidial plates begin to form, making their first 

 appearance as two minute triangular laminte, taking tlieir 

 origin in the basal angles of the foraminal triangle, and giv- 

 ing the foramen a lanceolate outline. 



By increments to their internal edges, these plates pres- 

 ently come in contact with each other, truncating the interior 

 basal angle of each, the plates being from this period onward 

 in progressive symphysis. The increments to these plates are 

 made more rapidly at and about their interior angles, and, as 

 a result, the foramen assumes successively an elliptical, an 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 ver- 

 tical, although the deltidial plates become slightly bent by 

 the incurving of the beak. A striking exception to this rule 

 is represented on Plate XIX, figure 9, where an individual 

 which has reached early maturity shows the senile feature of 



