Glossary of Morphological and Anatomical Terms

raduliform - Hook-shaped or rodlike crura that arise on ventral side of hinge plate and project toward ventral valve.
ramicostellate - Costellae numerous, arising entirely by branching.Fig.297.
ramulus - Folds developed on lateral walls of longitudinally divided dorsal median septum in thecideids.
receiving valve - See ventral valve.
rectimarginate - Having planar anterior commissure.Fig.289.
recumbent spines - Slightly curved spines extending at angle of less than 45° to shell surface.
reflexed interarea - Hypercline dorsal interarea.
remigrant - Pedicle opening that tends to move dorsally after initially migrating toward ventral beak.
resupinate - Reversal in relative convexity of postbrephic shells with convex ventral valve that becomes concave and with concave dorsal valve that becomes convex during successive adult stages of growth.Fig.287.
reticulate - Subrectangular patterns on shell exterior, commonly involving nodelike enlargements formed by intersection of concentric and radial elements of ornament.
retractor muscles - See anterior lateral oblique muscles.
rhizoid spines - Spines of productides resembling rootlets, serving for attachment either by cementation to substrate or by entanglement.
rhynchonellid stage - See uniplicate.
ribs - Any ornament of radial ridges.
ridge - Relatively long narrow elevation of secondary shell, indicated to variable depth within underlying floor of valve by low, wide deflections of the skeletal fabric normal to long axis of ridge.
rimmed - See auriculate.
ring - Precursor to ascending elements of terebratellide loop arising from resorption of hood apex consisting of thin circular ribbon; narrow ventrally and broadening dorsally to its attachment on septal pillar.
rostellum - Low projection between anterior adductor muscle scars of ventral valve of some cranioids to which internal oblique muscles are attached.
rostral callosity - See delthyrial plate.
rostrate - With prominent beak of ventral valve projecting over narrow cardinal margin.
rostrum - Elevation of secondary shell on inner surface of dorsal valve of some cranioids in front of anterior adductor muscles consisting of pair of low clubshaped protuberances forming seat of attachment for brachial protractor muscles.
ruga - Concentric or oblique wrinkling of external shell surface.Fig.294.