Cirsium is a genus of flowering plants in the family Asteraceae.  It is one of many genera with the common name being thistles.  Other genera known as thistles include Carduus, Cynara, Onopordon, and Silybum.

They are mostly native to Eurasia and northern Africa.  There are about 60 species in North America (although many species have been naturalized outside their native ranges).

Most species are considered weeds.  Cirsium vulgare is listed as a weed in nine US states.  Some species are planted in gardens for their beauty and to attract butterflies.

Species
Cirsium acarna - Soldier Thistle
Cirsium acaule - Stemless Thistle, Dwarf Thistle
Cirsium altissimum - Roadside Thistle, Tall Thistle
Cirsium amblylepis - Mt. Tamalpais Thistle
Cirsium andersonii - Anderson's Thistle, Rose Thistle
Cirsium andrewsii - Franciscan Thistle
Cirsium araneans - Jeweled Thistle
Cirsium arcuum - Powderpuff Thistle
Cirsium aridum - Desert Thistle
Cirsium arizonicum - Arizona Thistle
Cirsium arvense - Creeping Thistle, Field Thistle, Canada Thistle
Cirsium arvense var.argenteum
Cirsium arvense var. integrifolium
Cirsium arvense var. mite
Cirsium arvense var. vestitum
Cirsium barnebyi - Barneby's Thistle
Cirsium brachycephalum
Cirsium brevifolium - Palouse Thistle
Cirsium brevistylum - Clustered Thistle
Cirsium calcareum - Cainville Thistle
Cirsium californicum - California Thistle
Cirsium callilepis - Fringebract Thistle
Cirsium campylon - Mount Hamilton Thistle
Cirsium canescens - Platte Thistle, Prairie Thistle
Cirsium canovirens - Graygreen Thistle
Cirsium canum - Queen Anne's Thistle
Cirsium carolinianum - Carolina Thistle, Soft Thistle
Cirsium caulescens
Cirsium centaureae
Cirsium chellyense - Queen Thistle
Cirsium chuskaense - Monarch Thistle
Cirsium ciliolatum - Ashland Thistle
Cirsium clavatum - Lake Thistle
Cirsium clokeyi - Charleston Mountain Thistle, Whitespine Thistle
Cirsium congdonii - Rosette Thistle
Cirsium costaricense
Cirsium coulteri - Coulter’s Thistle
Cirsium crassicaule - Slough Thistle
Cirsium cymosum - Peregrine Thistle
Cirsium diacanthum
Cirsium discolor - Field Thistle, Pasture Thistle
Cirsium dissectum - Meadow Thistle
Cirsium douglasii 
Cirsium drummondii - Dwarf Thistle
Cirsium durangense
Cirsium eatonii - Eaton's Thistle
Cirsium edule - Edible Thistle
Cirsium engelmannii - Engelmann Thistle, Engelmann's Thistle
Cirsium eriophorum - Woolly Thistle
Cirsium erisithales - Yellow Melancholy Thistle
Cirsium esculentum
Cirsium flodmanii - Flodman Thistle, Flodman's Thistle
Cirsium foliosum - Drummond's Thistle, Elk Thistle, Leafy Thistle, Meadow Thistle
Cirsium fontinale - Fountain Thistle
Cirsium gilense - Gila Thistle
Cirsium grahamii - Graham's Thistle
Cirsium griseum - Gray Thistle
Cirsium hallii - Hall's Thistle
Cirsium helenioides - Melancholy Thistle, Common Melancholy Thistle
Cirsium heterophyllum - Melancholy Thistle
Cirsium hillii - Hill's Thistle
Cirsium hookerianum - White Thistle
Cirsium horridulum - Yellow Thistle
Cirsium humboldtense - Humboldt County Thistle
Cirsium hydrophilum - Suisun Thistle
Cirsium hypoleucum
Cirsium inornatum - Cloudcroft Thistle
Cirsium japonicum - Japanese Thistle
Cirsium kamtschaticum - Kamchatka Thistle
Cirsium lanatum - Hairy Thistle
Cirsium laterifolium - Porcupine Thistle
Cirsium lecontei - Le Conte's Thistle
Cirsium leo
Cirsium libanoticum
Cirsium loncholepis - LaGraciosa Thistle
Cirsium longistylum - Longstyle Thistle
Cirsium maritimum
Cirsium mendocinum - Mendocino Thistle
Cirsium mexicanum - Mexican Thistle
Cirsium modestum - Lacy Thistle
Cirsium mohavense - Mojave Thistle
Cirsium murdockii - Murdock's Thistle
Cirsium muticum - Swamp Thistle
Cirsium navajoense - Navajo Thistle
Cirsium neomexicanum - Lavender Thistle, New Mexico Thistle
Cirsium nipponicum
Cirsium nuttallii - Nuttall's Thistle
Cirsium occidentale -Cobweb Thistle
Cirsium ochrocentrum - Yellowspine Thistle
Cirsium oleraceum - Cabbage thistle
Cirsium olivescens - Summer Thistle
Cirsium oreophilum - Crow Thistle
Cirsium osterhoutii - Osterhout's Thistle
Cirsium ownbeyi - Ownbey's Thistle
Cirsium pallidum - Pale Thistle
Cirsium palustre - Marsh thistle, European Swamp Thistle
Cirsium parryi - Parry's Thistle
Cirsium pastoris - Snowy Thistle
Cirsium peckii - Steen Mountain Thistle
Cirsium pendulum
Cirsium perplexans - Rocky Mountain Thistle
Cirsium pitcheri - Pitcher's Thistle, Sand Dune Thistle
Cirsium praeteriens - Palo Alto Thistle, Lost Thistle
Cirsium proteanum - Red Thistle
Cirsium pulcherrimum - Wyoming Thistle
Cirsium pumilum - Pasture Thistle
Cirsium pyrenaicum
Cirsium quercetorum - Alameda County Thistle
Cirsium remotifolium - Fewleaf Thistle
Cirsium remotifolium var. remotifolium - Fewleaf Thistle
Cirsium repandum - Sandhill Thistle
Cirsium rhaphilepis
Cirsium rhothophilum - Surf Thistle
Cirsium rivulare
Cirsium rothrockii - Rothrock's Thistle
Cirsium rusbyi - Rusby's Thistle
Cirsium rydbergii - Rydberg's Thistle
Cirsium scabrum - Rough Thistle
Cirsium scapanolepis - Mountain Slope Thistle
Cirsium scariosum - Meadow Thistle
Cirsium scopulorum - Mountain Thistle
Cirsium segetum
Cirsium serrulatum
Cirsium spinosissimum
Cirsium subniveum - Gray Thistle, Jackson Hole Thistle
Cirsium tanakae
Cirsium texanum - Texas Thistle
Cirsium tioganum - Stemless Thistle
Cirsium tuberosum - Tuberous Thistle.  Similar to Cirsium dissectum (Meadow Thistle).
Cirsium turneri - Cliff Thistle
Cirsium undulatum - Gray Thistle, Wavy-leaf Thistle, Wavyleaf Thistle
Cirsium undulatum var. tracyi - Tracy's Thistle, Wavyleaf Thistle
Cirsium undulatum var. undulatum - Wavyleaf Thistle
Cirsium validus - Graceful Thistle
Cirsium vernale - Spring Thistle
Cirsium vinaceum - Sacramento Mountain Thistle, Sacramento Mountains Thistle
Cirsium virginense - Virgin Thistle
Cirsium virginianum - Virginia Thistle
Cirsium vulgare - Bull Thistle, Spear Thistle, Common Thistle, Fuller's Thistle (syn. C. lanceolatum)
Cirsium wheeleri - Wheeler's Thistle
Cirsium wrightii - Wright's Thistle
Hybrids
Cirsium × canalense - Canal Thistle
Cirsium × crassum - Thistle
Cirsium × erosum - Glory Thistle
Cirsium × iowense - Iowa Thistle
Cirsium × vancouverense - Vancouver Thistle

References

Asteraceae