330 



THE PALEONTOLOGY OF MINENSOTA. 



[Terminology. 



of Dr. Beecher,* is often present in the dorsal valve covering the car- 

 dinal process, but its development does not begin until early nealogic 

 or later growth, and is probably secreted by the dorsal mantle lobe. When 

 the delthyrium of the ventral valve contains two pieces growing out from 

 its walls, more or less uniting medially, they are called "deltidial plates", 

 and are deposited by the extensions from the ventral mantle lobe (see 

 d. p., fig. 23&). These plates are seen in the rhynchonelloids, spire and loop 

 bearing genera. The pedicle opening in these forms is always situated 

 above the deltidium or deltidial plates. 



B 



dldr 



v. ~~~ 



FIG. 22. a, interior view of the dorsal valve of Orthis occidentalis Hall; 6, interior view of the 

 ventral valve of Orthis insculpta Hall. 



c. ., cardinal area. 



card, p., cardinal process ; this may be a thin, simple plate, or thickened and trilobed, 

 or separated in two processes, as in the strophomenoids. 



dent, s., dental sockets ; the cavities into which the teeth of the ventral valve enter. 



cm. p., crural plates, usually forming the inner walls of the dental sockets, to which 

 the brachia are attached. 



p. add. and a. add., posterior and anterior adductor scars. 



o., genital [ovarian] spaces. 



m. Sep., median septum separating the two pairs of adductor scars. 



del., delthyrium, a term introduced by Hall (Pal. N. Y. vol. viii) for the trian- 

 gular space usually covered by the deltidium or deltidial plates; " fissure" 

 and "foramen" have also been used. 



c. a., cardinal area. 



o., genital [ovarian] spaces. 



t., teeth ; when they are supported by thin plates, the latter are termed "den- 



tal plates." 



* For a synopsis of the early embryology of the brachiopods. and a complete discussion of the development of the delti- 

 dium and deltidial plates, and on the term cbilidium, see " Development of the Brachiopoda, part 11, On the Stages of 

 Growth and Decline." American Jour, of Science. August, 1892. 



