BRACHIOPODA. 4fi3 
lihyricliotreiiia I'apiix.l 
opening under its apex [for the protrusion of the pedicle; it is formed by the deltidial 
plates, which grew from the walls of the delthyrium and joined medially, leaving an 
oval or circular aperture apically] in young examples; mesial sinus deep and well 
defined in gibbous specimens and less so in the young and more compressed forms, 
never quite reaching the front of the beak and always having three simple [some- 
times four], rather angular plications in the bottom that extend, like the others, to 
the apex of the beak in well preserved specimens; lateral slopes each occupied by from 
five to seven simple plications." Interior with prominent hinge teeth, supported, 
according to the age of the specimen, by more or less thickened dental plates which 
join the outer elevated margin of the deeply excavated pear-shaped muscular area. 
Posterior to the center of this area there is, in old examples, a deep, elongate depres- 
sion containing the adductor scars, and surrounding these are the large pear-shaped 
diductors, the adjustors being placed postero-laterally to the latter. Posterior to, 
and above the muscular area, and between the hinge teeth in old or very obese 
examples, there is a rather deep rostral cavity, which seems to have been largely 
produced by the apex of the dorsal valve having been forced in that direction by 
anterior shell growth. In Wisconsin examples this cavity is often crossed by the 
coalesced, concave deltidial plates, leaving under it a narrow pasage for the peduncle 
to be extruded through the umbo of the valve. 
"Entire surface of both valves marked by numerous very regular, strongly zigzag, 
prominent, sublaminar marks of growth, that become nearly or quite obsolete, some- 
times on old examples.'' (Meek, op. cit) 
Obese specimens of this species are usually found with the apex of the ventral 
valve more or less worn away. This is nearly always ascribed to imperfect preser- 
vation, or due to weathering. Ohio specimens in which the delicate, sublaminar 
growth lines are well preserved also have the apex more or less broken. The 
writers, therefore, conclude that, as the pedicle opening was encroached upon by 
the dorsal umbo, owing to the shell becoming more convex with age, the peduncle 
was forced back through the beak of the ventral valve. Sometimes portions of the 
entire beak are wprn away from the same cause. This condition is also seen in 
many species of both fossil and recent terebratuloids and may be due to convexity 
of the valves or to shortness of the peduncle. 
R. capax is often confounded with R. increbescens=^R. inaquiralvis Castelnau, but 
the larger size of the former, together with the greater convexity and thickness of 
the valves, will readily separate it from the latter. Even the young of R. capa.r can 
be distinguished from the adult of R. inmqiiivali-is by the obsolete fold and sinus, 
fewer and larger plications, greater transversity and more prominent subimbricating 
growth lines. 
