THE BRACHIOPOD SHELL. 305 



Botsfordia cselata (PI. LIX, figs, le, If, and Ik) the cardinal areas are present but subordinate 

 as compared with the areas of Oholella. They are still less in evidence in Botsfordia granulata 

 (PL LVII, figs. 4g-m) and Botsfordia pulchra (PI. LXII, figs. 5g-j). In Neoholus (PI. LXXXI, 

 figs. 2e-h) the reduction of the cardinal areas has progressed until they are no longer a marked 

 feature of either valve. In Trematoiolus (PI. LXXXIII and PL LXXXIV, figs. 5a and 5b) the 

 development, of the pedicle tube is much more advanced than in Oholella, but the cardinal areas 

 are here again a prominent feature; they are less clearly defined in Yorlcia (PL LXXXII, 

 figs, le-h), in which the pseudodeltidium of the ventral valve is developed (PL LXXXII, 

 figs. Ib-c). In other forms of the Siphonotretidse the cardinal areas as developed in the 

 Obolidje are not seen in the adult stages of growth. 



It is interesting to note that the vertical thickened cardinal area is present in the oldest 

 known bracliiopod, Trematoiolus excelsis (PL LXXXII, fig. 8), wliich occurs stratigrapliically 

 lower down in the Lower Cambrian of North America than any other species of bracliiopod 

 known to me. Linnarssonella, with its low false area and stronglj'' marked interior of its valves, 

 has also a well-defined cardinal area in some species {L. modesta, PL LXXVIII, fig. 8e; 

 L. girtyi, PL LXXIX, figs. 1 1, Im, and Ir), in tliis feature suggesting that it is more primitive 

 than either Acrotreta or Acroihele. Queiecia is a very ancient form, but so far as known it 

 passed in the adult beyond the stage of having a vertical cardinal area. Notloing is known 

 of the interior of Discinolepis of the Middle Cambrian, but in the dorsal valve of Keyserlingia 

 the tliickened cardinal area is a well-developed feature (PL LXXXI, fig. 4e). Acrotreta occa- 

 sionally shows a vertical cardinal area in the dorsal valve (A. deflnita, PL LXIV, fig. 2d; A. 

 Icutorgai, PL LXV, figs. 3h-j ; A. curvata, PL LXVIII, figs. Ik and 1 1; J.. ojMrensis, PL LXXIV, 

 fig. If; A. ophirensis descendens, PL LXXVIII, fig. Ic; and A. marjumensis, PL LXXVIII, 

 fig. 2c). Other examples doubtless occur, but it appears that in both Acrotreta and Acrothyra 

 the vertical cardinal areas are absent or so reduced as no longer to be a generic character; the 

 same is to an even greater degree true of Acroihele. Acrothele bellula (PL LVIII, figs. 5c-h) has a 

 defined cardinal area in the dorsal valve and a trace of one on the ventral valve; otherwise, 

 only the false external area back of the pedicle opening is known to me among the species of 

 Acrothele, its subgenus Redlichella, or Schizopholis. Nothing is known of a thickened vertical 

 cardinal area in Orhiculoidea or Philhedra. 



PROTREMATA. 



Two of the earliest forms of the Protremata (Billingselld (PL LXXXV) and Nisusia (PL O) 

 both have a clearly defined cardinal area on the ventral valve, divided by a delthyrium that is 

 more or less completely covered by a deltidium. These characters appear to be developed in 

 the same manner as in the Inarticulata; in other words, they are the result of holoperipheral 

 growth and pedicle pressure bulging the median region of the cardinal area. No deltidial 

 plates are known in the BillingsellidEe. So far as known Otusia has an open delthyrium. The 

 cardinal area of Wynnia (PL LXXXIX, fig. 4b) has a large, open delthyrium, with no trace 

 of a deltidium in any of the four specimens from India. The figure shows a bit of shell at the 

 top of the delthyrium, but this is not in place. The identification of the subfamily Eafinesquinse 

 by Eostrophomena is of doubtful value (PL XCV, figs. 6, 6a-b) and no tiling is known of the 

 area of the ventral valve. Eoorthis (PL XCI),- Orusia (PL XCVIII), and Finkelnburgia 

 (PL XCIII) have an open delthyrium in the ventral valve. The cardinal area of Swantonia 

 (PL CIV, figs. 5 and 6) is unknown. Huenella has a clearly defined area on both valves and an 

 open delthyrium (PL CHI, figs. Ic, 2g, 2i, and 3"). 



BELTHYRIUM AND DELTIDIUM. 



The development of the delthyrium from the simple Rustella stage, where it is scarcely 

 more than a broad opening between the valves, to the definite form in the higher types of the 

 Protremata, where it is either open or more or less closed by a deltidium, has been outlined in 



62667°— VOL 51, pt 1—12 20 



