McLean & Geiger:
Shell height moderate; anterior slope short, sometimes concave; protoconch and short selenizone present only on juvenile shell posterior to foramen; protoconch with linear and concentric sculpture; expansion of foramen obliterates protoconch with growth. Foramen bordered posteriorly on inner surface by a broad, truncated callus. Sculpture radial and concentric, radial sculpture marked by primary and secondary ribs. Mantle skirt intact anteriorly. Epipodial tentacles numerous, of similar size. Ctenidia bipectinate, gill axis free. Rachidian tooth broad to narrow, inner lateral teeth narrow; pluricuspid tooth massive.

Shell cone-shaped; aperture oval; hole small or moderately large, situated somewhat anteriorly to the center; apex of the adult shell rudimentary; surface with finer or coarser radial ribs and in most cases with concentric folds; on the inner side the hole is surrounded by a posteriorly truncated callus; margin at the ends of the ribs pleated, and in one plane or laterally raised.
Animal is retractable into the shell; edge of mantle smooth or pustulate; epipodium consisting of a series of small tentacles. Central plate of the radula as broad as long or broader than long, the remaining plates similar to those in Emarginula.

Shell conical, elevated or depressed, the size varying between wide limits. Apex in front of the shell center: in young specimens curved and slightly twisted, inclined backward and with a fissure in front of it. In the adult, this small and slightly twisted portion is totally lacking. Orifice at the summit or immediately anterior to it, variable in shape. Surface of the shell sculptured by conspicuous radiating ribs and concentric threads or laminae. Basal margin of the shell resting upon a plane or slightly elevated in the middle part of the sides; never raised at the ends. Margin strongly crenulated. Muscle impression frequently indistinct, but when distinct, not uniform in width and with the anterior part incurved at the ends. A small and narrow connecting ridge bridges the area between the incurved ends. A few species have the muscle scar completely oval and of a uniform width. The orifice is bounded by a callus which is truncated posteriorly and sometimes excavated at this region forming a deep pit. Shell completely external, not covered by the mantle and capable of containing all the soft parts.
The animal possesses one pair of tentacles with the eyes on little projections on their outer bases. Snout short, oval and disc-shaped with the mouth in the middle of the disc.
Pilsbry states "epipodium consisting of a fringe of short filamentous processes, alternately larger and smaller, becoming more obsolete posteriorly where it is marked by little tubercles in place of the filaments. Mantle-edge smooth or papillose, usually broad". The central tooth of the radula in Diodora is large, while in Fissurella it is very small.

Sculpture cancellate; callus inside apex squared posteriorly; muscle scars with hooked ends.

In this genus the septum is reduced to a posteriorly truncate ridge of callus. It also differs from other genera treated here in having the anterior slope rather than the posterior slope shorter and sometimes concave, although this is shared with Altrix.

Comparing Fissurella with Diodora:

Fissurella:
• The orifice the same width throughout;
• Sometimes with its basal margin raised at the ends.
• Central tooth of the radula narrow.

Diodora:
• Shell with the internal callus of the orifice truncated and frequently excavated posteriorly:
• With its basal margin never raised at the ends.
• Central tooth of the radula wide.