88 MEMOIRS OF THE STATE MUSEUM 



generate the normal deltidial advancement, the plates becoming thickened 

 and their features obscured, while in some species processes .are given off, 

 as in a number of the Mesozoic Rhynchonellidse. 



The cardinal area of the Strophomenidse in its early phase shows a 

 small pedicle-sheath for the ventral valve and a narrow grooved process 

 under the beak of the dorsal. The perforation for the passage of the 

 peduncle does not materially increase in size with the growth of the shell 

 and often is obliterated, while the dorsal callosity usually reaches a consid- 

 erable development. 



Additional evidence of the degeneracy of the pedicle is afforded by 

 many species of other genera, which have a calcareous attachment to for- 

 eign objects at the apex of the ventral valve, the pedicle, therefore, becom- 

 ing functionally obsolete. 





 Observations having some analogy with the facts here presented have 



been made, in a very restricted sense and usually incidentally, by various 

 authors. Our own results, though derived from the species of a single 

 fauna, must not be given too limited an application, for they involve nearly 

 every important family of paleozoic articulate brachiopods, and we may 

 tentatively assume that, as a rule, the essential features of variation ob- 

 served in any member of a genus will hold good of the other members. 

 In regard to the development of the characters of the pedicle-passage, i. e. 

 the deltidial plates and the foramen, there is good reason to regard the 

 process as substantially identical in all the genera represented, making the 

 necessary allowance for the peculiar variation seen in the Strophomenidse, 

 which may not, however, prove it an exception to the general statement. 



The various terms which have sometimes been applied to the condition 

 of the deltidium in the rostrate genera, as deltidium amplectcns, when the 

 foramen is entirely surrounded by the plates, as in various Mesozoic Rhyn- 

 chonellae (but in no paleozoic species of which we are aware), deltidium sec- 

 tans, when the plates bound the foramen only on the lower side, the upper 



