SILURIAN BRACHIOPODA 79 



by Deslpngchamps (see Summary), and the characteristic mode of develop- 

 ment in Tcrcbratula and RJiynchonella. 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 Rhynclwnclla. Spirifer 

 niagarensis, S. perlamellosus, and . cumberlandice are examples of the former 

 mode, and S. sulcatus and approximately S. radiatus represent the latter. 

 Both conditions are reached by accretion along the inner edges of the del- 

 tidial plates. The initial state is represented by a narrow elongate lamina 

 on each side of the triangular area. Further growth produces a triangular 

 plate t and to the form of the triangle is due the apparent growth of the 

 deltidium from the apex of the fissure downwards, or from the base of the 

 fissure towards the beak of the ventral valve. 



The accompanying diagrammatic outlines serve to illustrate the 

 changes and the final results. 



X 



r 



FIG. 4. DEI.TIDIAL DEVELOPMENT IN SPIRIFER. 



Figure*i represents an area in an early stage of growth, with a narrow 

 deltidial plate on each side, alike for each series. 



Figure 2 shows scalene triangular plates, with the shortest side at the 

 base of the area. 



Figure 2' shows plates with the two free edges more nearly equal. 



