DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 385 



Figures 4, 4', 4" show the completed deltidial plates with 

 the circular perforation. 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 posi- 

 tion 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 pseudo-deltidium is also 

 the same, although the methods of attaining the result differ 

 in each. 



Figures 1, 2, 3, 4 are represented by S. sulcatus,* and vary 

 in no important particulars from the mode of development in 

 Terebratula and Rhynchonella. 



Figures 1, 2', 3', 4' are partially represented by S. radiatus, 

 although in this species the circular foramen is usually oblit- 

 erated by subsequent thickening and growth. (See figures 

 9, 10, 11, Plate XX.) 



Figures 1, 2", "3" are well shown in S. niagarensis (figure 

 8, Plate XX), and the subsequent stages appear in mature 

 forms of S. perlamellosus and S. cumberlandice. Other forms, 

 notably those with elevated areas (such as S. macronotus, 

 S. medialis, together with Cyrtina and Cyrticf), present con- 

 siderable differences in the completed pseudo-deltidium, due 

 principally, it is believed, to the internal thickening of the 

 beak and the growth of the transverse septum. 



The pseudo-del tidium of Spirifer thus appears to be the 



* State of New York, Report of the State Geologist for the year 1882, pub- 

 Hshed 1883, pi. ix, figs. 1, 2, 3. 



25 



