DEVELOPMENT OF THE BRACHIOPODA 263 



border limiting the deltidial opening and passing along the 

 sides of the pedicle at its base. The ventral mantle grad- 

 ually extends from each side as two prolongations partially 

 covering the opening and enveloping the proximal portion of 

 the pedicle. As this is an extension of the shell-secreting 

 surface of the mantle, there naturally results the formation 

 of two plates within the deltidial area. Their structure is 

 commonly punctate whenever the valves are punctate. 



These outgrowths or extensions of the mantle into the del- 

 tidial area finally touch and coalesce until, as in M. Jlavescens, 

 the pedicle emerges through an opening in the ventral mantle, 

 and pari passu the deltidial plates unite and limit the pedicle- 

 opening to the beak of the ventral valve. The latter process 

 has been carefully described by Deslongchamps,^ Clarke, and 

 the writer,-^ and need not be dwelt on here. Figures 119 and 

 120 of the beaks of T. septentrionalis and M. fiavesceiis with 

 the shell removed show the relations of the ventral mantle 

 to the pedicle, and the portions which secrete the deltidial 

 plates. 



The deltidium and delthyrium are often simulated in the 

 growth of the dorsal valve in genera having a high cardinal 

 area in this valve. Orthis, Leptcena, Clitambo7iites, Spirifer^ 

 and StricMandinia may be cited as examples. They cannot 

 be properly correlated with similar parts in the ventral valve, 

 for their origin is quite different. Primarily, a deltidial 

 opening is for the extrusion of the pedicle, and this belongs 

 properly to the ventral valve. The dorsal fissure is the space 

 between the diverging teeth sockets, and may be filled by the 

 cardinal process, as in Leptcena and Orthis, or it may have 

 in addition a convex plate or chilidium covering it, as in 

 Clitamhonites. In Spirifer and Stricklandiyiia the opening 

 remains unclosed. 



The true deltidial plates are formed on the side of the 

 pedicle adjacent to the hinge by extensions of the ventral 

 mantle lobe, and begin as two plates. They are likewise 

 expressive of maturity, and are of secondary development, 

 while the deltidium begins as a single plate in the median 



