C. E. Beecher — Development of the Brachiopoda. 147 



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 punc- 

 tate whenever the valves are punctate. 



These outgrowths or extensions of the mantle into the delti- 

 dial area finally touch and coalesce until, as in M.flavescens, the 

 pedicle emerges through an opening in the ventral mantle, 

 ;md 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, 6 Clarke and 

 the writer 3 and need not be dwelt on here. Figures 35 and 

 36 of the beaks of T. septentrionalis and M. flavescens 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, Leptsena, Clitambonites, Spirifer, 

 and Stricklandinia, may be cited as examples. They cannot 

 properly be correlated with similar parts in the ventral valve, 

 for their origin is quite different. Primarily, a deltidial open- 

 ing is for the extrusion of the pedicle and this belongs pro- 

 perly 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 Leptsena and Orthis, or it may have in 

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

 bonites. In Spirifer and Stricklandinia, the opening remains 

 unclosed. 



The true deltidial plates are formed on the side of the pedi- 

 cle 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 line, and is 

 eminently a primitive character in the Protremata. 



From present knowledge of the group, it is difficult to offer 

 an explanation for the presence of an anal opening in the 

 Inarticulata and its absence in the recent Articulata, as the 

 solution of the question depends upon whether the class is to 

 be considered as progressive or degraded. The dorsal beaks of 

 Amphigenia, Athyris, Cleiothyris, Atrypa, and Rhynchonella, 

 are usually notched or perforate. The perforation comes from 

 the union of the crural plates above the floor of the beak leav- 

 ing a passage through to the apex. A similar opening occurs 

 between the cardinal processes in Strophomena, Stropheodonta, 

 and allied genera, and the chilidium may also be furrowed, as 

 in Lejjtama ( — Strophome?ia) rhomboidalis. This character 

 is evidently in no way connected with the pedicle opening, 



Am. Jour. Sci.— Third Series, Vol XLIV, No. 260.— August, 1892. 

 10 



