Editing Hyolitha (section)

==Morphology==
The shell of a hyolith is typically one to four centimeters in length, triangular or elliptical in cross section. Some species have rings or stripes. It comprises two parts: the main conical shell (previously referred to as a ‘conch’) and a cap-like [[Operculum (animal)|operculum]]. Some also had two curved supports known as ''helens''<ref name="ReferenceA"/> They are [[calcium carbonate|calcareous]] &ndash; probably aragonitic<ref name="Mus2007">{{Cite journal|pages= 1231–1243|doi=10.1111/j.1475-4983.2007.00700.x|title=Skeletal Microstructure of Helens, Lateral Spines of Hyolithids|journal=Palaeontology|volume=50|issue=5|date=September 2007|last1=Mus|first1=M. Martí|last2=Bergström|first2=J.|doi-access=free}}</ref> All of these structures grew by marginal accretion.

=== Shell microstructure===
The orthothecid shell has an internal layer with a microstructure of transverse bundles, and an external layer comprising longitudinal bundles.<ref name="Mus2007" />

=== Helens ===
Some hyoliths had helens, long structures that taper as they coil gently in a [[logarithmic spiral]] in a ventral direction.<ref name="Mus2007" /><ref name="ReferenceA"/> The helens had an organic-rich central core surrounded by concentric laminae of calcite.  They grew by the addition of new material at their base, on the cavity side, leaving growth lines.<ref name="Mus2007" />

They were originally described by [[Charles Doolittle Walcott|Walcott]] as separate fossils under the genus name ''Helenia'', (Walcott's wife was named Helena and his daughter Helen); [[Bruce Runnegar]] adopted the name helen when they were recognized as part of the hyolith organism.<ref name="Mus2007" /> Encrusting organisms have been found on helens, and also on both sides of the main shell, all of which are therefore supposed to have been raised above the sea bed. The helens have been interpreted as props that supported the feeding organ, the [[lophophore]], above the sea bed.<ref name="ReferenceA">{{cite journal|last1=Moysiuk|first1=Joseph|last2=Smith|first2=Martin R.|last3=Caron|first3=Jean-Bernard|title=Hyoliths are Palaeozoic lophophorates|journal=Nature|date=11 January 2017|volume=541|issue=7637|pages=394–397|doi=10.1038/nature20804|pmid=28077871|bibcode=2017Natur.541..394M |s2cid=4409157 |url=http://dro.dur.ac.uk/20195/1/20195.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://dro.dur.ac.uk/20195/1/20195.pdf |archive-date=2022-10-09 |url-status=live}}</ref>

=== Operculum ===
The operculum closes over the aperture of the shell, leaving (in hyolithids) two gaps through which the helens can protrude.<ref name="Mus2007" />  It comprises two parts: the cardinal shield, a flat region at the top of the shell; and the conical shield, the bottom part, which is more conical.<ref name="Mus2007" />  The inside of the shell{{fix|text=the inside of the operculum?}} bears a number of protrusions, notably the dorsal cardinal processes and the radially-arranged clavicles.<ref name="Mus2007" />
[[File:Haplophrentis.png|thumb|Reconstruction of ''[[Haplophrentis]]'', hyolith with known soft tissue]]

===Soft tissues===
The soft tissues of the mid-Cambrian hyolith ''Haplophrentis'', from the [[Burgess Shale]] and [[Spence Shale]] [[Lagerstätten]] include a gullwing-shaped band below the operculum. This band is interpreted as a lophophore, a feeding organ with a central mouth; it bears 12 to 16 tentacles. From the mouth a muscular pharynx leads to a gut, which loops back and exits beyond the crown of tentacles. Next to the gut are a pair of large kidney-shaped organs of uncertain nature. Under the operculum are muscles. The thin body wall circumscribes the interior of the shell, except the apex.<ref name="ReferenceA"/> Preserved intestines have been described from the Ordovician hyolith ''Girvanolithes thraivensis''.<ref name="Malinky2003">{{cite journal |last1=Malinky |first1=John M. |date=July 2003 |title=Ordovician and Silurian hyoliths and gastropods reassigned from the Hyolitha from the Girvan district, Scotland |url=https://www.cambridge.org/core/journals/journal-of-paleontology/article/abs/ordovician-and-silurian-hyoliths-and-gastropods-reassigned-from-the-hyolitha-from-the-girvan-district-scotland/3B5962257238EC74BB2CA143B69AB2C0 |journal=[[Journal of Paleontology]] |volume=77 |issue=4 |pages=625–645 |doi=10.1666/0022-3360(2003)077<0625:oashag>2.0.co;2 |access-date=26 November 2022}}</ref>