BRACHIOPODA. 105 



Shell elongate-oval, rather broad over the pallial region. Surface covered 

 with rather coarse, angular, usually simple plications. There is a trace of an 

 indistinct median sinus on the pedicle-valve in which the plications are slightly 

 smaller than those adjoining. The umbo of the pedicle-valve is incurved and 

 its apex truncated by a circular foramen. The deltidium is triangular, flat, or 

 arched by the incurvature of the beak ; the deltidial plates are tirmly anchyl- 

 osed into a single piece and the original line of symphysis is represented by a 

 thickened ridge. The edges of the cardinal area are well defined, but not alate 

 on either valve, the beak and area of the brachial valve being entirely concealed 

 by incurvature. The cardinal slopes are broad and smooth. On the interior of 

 the pedicle-valve the teeth are rather small, and are supported by thin lamellae 

 which traverse the umbonal cavity and rest on the bottom of the shell. These 

 lamellae are produced forward for a short distance, limiting, posteriorly, the 

 muscular area. The apical portion of the umbonal cavity contains a longitud- 

 inal tube attached by one side to the inner surface of the deltidium. Just 

 within the outer opening of the foramen this tube appears to have been closed 

 on all sides, but further toward the cardinal margin it becomes split along the 

 back or outer surface, diminishing in size downward and disappearing entirely 

 before reaching the hinge-line. (In the accompanying figures of transverse 

 sections, 80-83 are from a single specimen, 84, 85 from another, and 86-98 

 from a third.) In sections made across the vertical foramen it is seen that 

 the tube extends within the deltidium and forms a subcircular enfolding of 

 testaceous matter from the margins of the foramen. This organ is similar to 

 that elsewhere described in the genera Hostedia and Acambona, but it appears 

 to be more highly developed and longer in Retzia than in either of these. 



