Editing Hyolitha (section)

== Taxonomy ==
{{See also|List of bilaterial animal orders}}
[[Image:Hyoliths02.JPG|thumb|250px|Hyoliths from the Middle [[Ordovician]] of northern [[Estonia]]; these are internal molds.]]
The hyoliths are divided into two orders, the [[Hyolithida]] and the [[Orthothecida]].

Hyolitha have dorso-ventrally differentiated opercula, with the ventral surface of the shell extending forwards to form a shelf termed the ligula.<ref name="Mus2007" />

The Orthothecida are somewhat more problematic, and probably contain a number of non-hyoliths simply because they are so difficult to identify with confidence, especially if their operculum is absent.<ref name="Mus2007" />  They have a straight (planar) opening, sometimes with a notch on the bottom side, and sealed with an operculum that has no ligula, clavicles, furrow or rooflets.<ref name=Malinky2004/>

''Hyptiotheca'' is an unusual hyolithid, in that it lacks clavicles.<ref name="Malinky2004">{{Cite journal | author1=Malinky, J.M. | author2=Skovsted, C.B. | title = Hyoliths and small shelly fossils from the Lower Cambrian of North−East Greenland | journal =  Acta Palaeontologica Polonica |volume=49 |issue=4 |pages=551–578 | year=2004|url=http://app.pan.pl/article/item/app49-551.html}}</ref>

Orthothecids fall into two groups: one, the orthothecida ''sensu stricto'',<ref name="ref_a">{{Cite journal| last1 = Malinky | first1 = J. M.| title = First Occurrence of ''Orthotheca'' Novák, 1886 (Hyolitha, Early Devonian) in North America| journal = Journal of Paleontology| volume = 83| issue = 4| pages = 588–596| year = 2009| doi = 10.1666/08-164R.1| s2cid = 130227630}}</ref> is kidney or heart shaped in cross-section due to a longitudinal groove on its ventral surface, and its opercula bear cardinal processes; the other has a rounded cross-section and often lacks cardinal processes, making them difficult to distinguish from other cornet-shaped calcareous organisms.<ref name=Malinky2004/>  All were sessile and benthic; some may have been filter feeders.<ref name="ref_a" />