SILURIAN BRACHIOPODA 87 



and more incurved, until, finally, it lies entirely within the ventral umbonal 

 cavity. 



Those species furnished with a circular apical perforation, as Atrypa 

 reticularis, Retzia evax, and Rhynchotreta cuneata, lose the initial point of 

 the ventral beak from absorption, due to the increase in the size of the per- 

 foration or to its final terminal position. In Atrypa reticularis, or Whit- 

 ficldia maria even both beak and perforation are destroyed, from the 

 forcing of the ventral beak into contact with the dorsal umbo, produced by 

 the great increase in the depth of the valves from growth along their 

 anterior margins. 



Cardinal area. Omitting for the present the Strophomenidae 



and Orthidae, the initial state of the ventral cardinal area for all other forms 

 is a broad triangular opening beneath the beak, with simple sharp margins. 

 This condition is never passed by Meristella recttrostra, which shows a 

 uniform, open, triangular area in every period of growth. 



A farther advanced state of progress initiates thedeltidial plates, which 

 first appear as narrow laminae along the sides of the area. The areal de- 

 velopment of Spirifer crispus, Rhynchonella neglecta, and R. acinus, ceases 

 at this point. 



In the next stadium, the further growth of -the deltidial plates along 

 their free edges gives them a triangular form, and they tend to narrow the 

 limits of the opening and define the peduncular foramen. Spirifer niagar- 

 ensis and Rhynchonella whitii represent species which are arrested at this 

 period. 



The completed growth shows the deltidial plates uniting by symphysis 

 along a median line, and inclosing near the apex of the area a more or less 

 circular pedicle-perforation. Rhynchotreta cuneata, Whitfieldia maria, 

 Retzia evax, etc., after passing through all the earlier conditions, reach this 

 limit of development. 



The results of senile and extravagant growth often obliterate or de- 



