• Aarchaeocidaris strawberryensis Webster, 1997 †: Mississippian
• Abakana macrospina (Schoenmann, 1926) †: Givetian
• Abiella beloserskiensis Ivanova, 1963 †: Cisuralian
• Abiella chejagica Kanev, 1989 †: Roadian
• Abiella kolvae Kanev, 1983 †: Kungurian
• Abiella satura Kanev, 1994 †: Kungurian
• Acanthoceramoporella spinigera Ernst & Munnecke, 2009 †: Hirnantian
• Acanthoceramoporella triloba (Cumings & Galloway, 1913) †: Ordovician
• Acanthocidaris maculicollis (de Meijere, 1904): Cisuralian
• Acanthocladia anceps (Schlotheim, 1820) †: Permian
• Acanthocladia antiqua Nekhoroshev, 1956 †: Carboniferous
• Acanthocladia biserialis Chronic, 1949 †: Paleozoic
• Acanthocladia ciscoensis Moore, 1929 †: Pennsylvanian
• Acanthocladia crebriramosa Trizna, 1939 †: Permian
• Acanthocladia macer Trizna, 1950 †: Sakmarian
• Acanthocladia minor Korn, 1930 †: Permian
• Acanthocladia pampinosa Trizna, 1950 †: Sakmarian
• Acanthocladia regularis Bassler, 1929 †: Permian
• Acanthocladia striata Bolkhowitinova & Markoff, 1926 †: Carboniferous
• Acanthoclema confluens (Ulrich, 1888) †: Visean
• Acanthoclema distilum Bigey, 1988 †: Devonian, Middle
• Acanthoclema elegantula Ernst, 2008 †: Devonian, Middle
• Acanthoclema lineatum McNair, 1937 †: Devonian
• Acanthoclema ohioense McNair, 1937 †: Givetian
• Acanthoclema parvula Ernst, Dorsch & Keller, 2011 †: Eifelian
• Acanthoclema parvula Ernst, Dorsch & Keller, 2011 †: Emsian
• Acanthoclema sulcatum Hall & Simpson, 1887 †: Givetian
• Acanthoclema triangularis Ernst & Königshof, 2010 †: Givetian
• Acanthoclema triseriale (Hall, 1883) †: Emsian
• Acantholunaria expansa Ernst, Munneke & Oswald, 2015 †: Wenlock
• Acanthostictoporella angusta Ernst & Königshof, 2010 †: Givetian
• Acanthostictoporella modesta Ernst, 2011 †: Devonian, Lower
• Acanthotrypa carnica Vinassa de Regny, 1915? †: Ordovician
• Actinodonta acuta Barrois, 1891 †: Floian
• Actinodonta cuneata Phillips, 1848 †: Llandovery
• Acupipora mexicana Ernst & Vachard, 2017 †: Kasimovian
• Adzvaella vegeta Kanev, 1994 †: Capitanian
• Aganaster gregarius (Meek & Worten, 1869) †: Devonian, Upper
• Aganaster huaanensis (Wu, 1980) †: Capitanian
• Aganaster qingchangensis (Feng, 1985) †: Changhsingian