8o MEMOIRS OF THE STATE MUSEUM 



Figure 2" presents narrow triangular plates as in figure 2, but with 

 the shortest edges in the apex of the area. 



In figures 3, 3', 3", the growth has continued in the direction initiated 

 in the preceding stage, and the apex of the area has been partially filled 

 from the internal thickening of the beak. 



Figures 4, 4', 4" show the completed deltidium with the circular perfo- 

 ration. The plates in figure 4 nearly close the area, while in figure 4" the 

 opening is nearly as high as wide. Further growth can now take place 

 only along the lower free edges of the plates. 



Figure 5 represents the results of subsequent growth and thickening, 

 which have obliterated the evidences as to the mode of development, 

 and unified all three cases. The position of the foramen below the apex of 

 the area does not appear to be due to the approximation and union of 

 the deltidial plates, but to the lowering of the actual cavity of the beak 

 from the natural thickening of the shell, so that the foramen, as in other 

 genera, is at the real termination of the ventral umbonal cavity. 



It is seen that the manner of development is alike in each case, varying 

 only from differences in the form of the plates in the earlier stages. The 

 finished deltidium is also the same, although the methods of attaining the 

 result differ in each. 



Figures r, 2, 3, 4 are represented by ^. sulcatus* and vary in no im- 

 portant particulars from the mode of development in Terebratula and 

 Rhynchonella. 



Figures i, 2', 3', 4' are partially represented by S. radiatus, although, in 

 this species, the circular foramen is usually obliterated by subsequent thick- 

 ening and growth. (Sec figures 10, 9, 1 1, plate 6.) 



Figures i, 2", 3" are well shown in S. niagarensis (figure 8, plate 6), and 

 the subsequent stages appear in mature forms of S. perlamcllosus and S. ciiin- 



* State of New York, Report of the State Geologist for the year 1882, published 1883, pi. 60, 

 figs, i, 2, 3. 



