90 MEMOIRS OF THE STATE MUSEUM 



are expressed by the outlines 2", 3", on page 79, and is further shown in 

 the accompanying illustration. Additional 

 growth causes the plates to unite along the 

 median line, obliterating the partially formed 

 pedicle-perforation, and subsequent increment 

 can naturally only take place along their lower 

 free edges. Fig. 4. Deltidial development in 



Our remarks on Sptrijcr ramatus and o. j,4. sfiri/rrinawahotn^soy^. 



y. Spiri/erina rostrata, Schl. 



crispus indicate that the development of the 



plates in this member of the same family is quite in harmony with the 



process as seen in the rostrate forms generally. 



The following is the summarization of Deslongchamps's conclusions as 

 given by himself : 



(i) The deltidium is one of the most important features in the articu- 

 lated brachiopods. 



(2) As far as Jurassic species are concerned, the deltidium may sufifice 

 to characterize the families. 



(3) In the various stages of development of this part, the aspect of the 

 shell is entirely changed. 



(4) The deltidium appears under three important modifications : A. 

 development below the peduncular arm, characterizing the Terebratulidae, 

 C. development above the peduncle, Spiriferidae, B. mixed development, 

 surroundiijg the peduncle, Rhynchonellidae. 



(5) The stage at which the development is arrested or the exuberance 

 of development may suffice to characterize sections under the families. 



It has just been shown that conclusions 2, 4, and 5 are not capable of 

 the extended application which he has given them. 



A preceding remark, that the course of development of the deltid- 

 ial characters throughout the genera here discussed, may be considered as 

 fundamentally uniform, calls for explanation in its application to Orthis and 



