DEVELOPMEXT OF SOME SILURIAN BRACIIIOPODA 383 



Developmental Changes. 



The changes in the shell from advancing growth are prin- 

 cipally the gradual widening of the valves, on account of the 

 extension of the cardinal line and extremities, and the in- 

 curving of the beaks, from the progressive increase in the 

 depth of the valves. From being circular in outline, the 

 shell slowly widens until it is one-seventh wider than long. 

 The ventral beak in old specimens is so arched over the area 

 as nearly to conceal it, and prevent the opening of the valves 

 to any extent. In the early stages the depth of the con- 

 joined valves is about half the length of the shell, while in 

 obese mature forms the depth is equal to the length. 



The deltidial plates first appear as narrow elevated laminse 

 along the sides of the fissure under the ventral beak. A 

 specimen about half-grown shows them as represented in 

 Plate XX, figure 9, consisting of triangular plates approxi- 

 mately as in figure 3' of the following diagram. They are 

 subsequently united along their inner margins, and rarely, 

 in the material at hand, can any appearance of a foramen be 

 discovered. In old shells the growth and thickening of 

 the pseudo-deltidium makes it rugose, and it nearly closes 

 the area. 



From an examination of a number of species of Spirifer 

 showing considerable variety in the mode of development of 

 the pseudo-deltidium, it is believed that there is no essential 

 difference, and that all intermediate conditions between the 

 features represented in Spirifcrina by Deslongchamps (see 

 Summary) and the characteristic mode of development in 

 Terebratula and Rhynchonella occur in this group. The genus 

 Spirifer presents all these stages. In some species the area 

 is apparently closed by growth from the apex, and in others 

 by the meeting of the deltidial plates at the base of the area 

 and inclosing a foramen as in Rhynchonella. Spirifer niag- 

 arengis, S. perlamellosus, and S. cumherlandiae are examples 

 of the former mode, and *S'. suloatus and approximately 

 ;S'. radiatus represent the latter. Both conditions are reached 



