MOLLUSCOIDEA — BRACHIOPODS 



195 



Crania (Fig. 76). Ordovician to present. 



Shell small, nearly circular in outline, without pedicle open- 

 ing in adult but cemented by apex or by entire surface of pedicle 

 valve to the surface of a shell or other foreign object. Brachial 

 (upper) valve conical, with beak 

 almost central and directed pos- 

 teriorly. In each valve there is a 

 pair of widely separated muscle 

 scars near the posterior margin 

 and a pair close together near the 

 center. Impressions of the pallial 

 sinuses branch finger-like. 



I. Sketch a valve, naming the 

 valve and noting muscle scars if 

 present. 



Sub-class 2, Articulata 



Pig. 76. — Crania bordeni Hall and 

 Whitfield, from the Middle De- 

 vonian of Indiana. Natural size. 

 A, an entire individual attached 

 to a brachiopod shell which has 

 vertical ribbing at the right of 

 the figure. The conformation of 

 the upper valve to the ornamen- 

 tation of the brachiopod shell in- 

 dicates the intimate relationship 

 existing between the mantle edges 

 of the two valves. B, side view 

 of the same. (After Hall and 

 Clarke.) 



Valves held firmly together by 

 teeth and sockets (whence the 

 name from Latin articulatus, 



jointed). In living species the intestine ends blindly, but in 

 some Paleozoic forms it apparently opened through a foramen 

 in the cardinal area of the brachial valve ; if this opening was 

 in life occupied by the anus, it suggests that a degeneration in 

 structure has accompanied the retrogression in numbers for this 

 sub-class. 



Rafinesquina (Fig. 77, A, B). Almost wholly Ordovician. 



Pedicle valve more or less convex, often without pedicle 

 opening in the adult. The apex of its beak has a minute de- 

 pression, — the pedicle opening of the very young shell, — 

 but with age the pedicle usually disappeared, probably through 

 resorption. The shell rested free upon the sea-bottom, but 

 was held in place through social crowding, while the inner end 

 of the pedicle opening became filled with lime, secreted prob- 



