The test ( or shell ) usually starts off streptospiral ( coiled irregularly ) hence the name, and becomes planispiral in the mature stage.
12.
For the rest, nautilids adapted the standard planispiral shell form, although not all were as tightly coiled as the modern nautilids ( Teichert 1988 ).
13.
Ancyloceratidae are derived from the Crioceratidae, a family of Lower Cretaceous ammonites with loosely wound, open planispiral shells, probably originating from within the suborder Lytoceratina.
14.
J . B . Knight, " et al . " ( 1960 ) regarded the bellerophontids as torted gastropods even though they have essentially symmetrical, planispiral shells.
15.
The microspheric test begins with a short planispiral stage, later chambers spreading successively from flabeilliform ( fan-shaped ) to reniform ( kidney-shaped ) and finally annular.
16.
A third complex consists of species with rather large, openly coiled to nearly planispiral shells with pronounced prickly sculpture and widely expanded outer lips; the shells are often strongly patterned.
17.
"' Planispiral "'is a condition in which a tubicolous shell is coiled in a single horizontal plane and the diameter increases away from the axis of coiling.
18.
The included genera are characterized by asymmetrical, semi-involute to partially evolute, aragonitic tests consisting of an undivided planispiral or trochospiral tubular chamber wound about the proloculus, or initial chamber.
19.
The word is also applied to the depressed central area on the planispiral coiled shells of " Nautilus " species and fossil ammonites . ( These are not gastropods, but shelled cephalopods .)
20.
"Annuloceras " is a heteromorph ammonite similar in form to other heteroceratids except that the early helix is more loosely wound . " Macroscaphites " differs in that the early portion is planispiral and tightly wound.
