Key to Infraorder Brachyura:  True Crabs

Taken primarily from  Kozloff, 1987, 1996  p. 411 (Copyright 1987, 1996, University of Washington Press.  Used in this web page by permission of University of Washington Press)

An alternate key to a similar group of families can be found in Wicksten, 2009 page 288.
 

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1a	Aggregate of mouthparts tapering anteriorly, thus forming a triangle (carapace broader than long, and with a large, laterally directed tooth originating at the widest part)	Family Calappidae
1b	Aggregate of mouthparts not tapering anteriorly	2
2a	With a prominent rostrum that is divided into 2 processes (photo) (these may be partly or wholly fused)	Family Majidae
2b	Without a rostrum (photo) (photo)	3
3a	Anterior margin of carapace, between the eyes, usually toothed (not toothed, however, in the family Xanthidae); lateral margins of carapace distinctly toothed (photo)	4
3b	Anterior margin of carapace, between the eyes, not toothed; lateral margins of carapace either toothed or not toothed	7
4a	Anterior margin of carapace, between the eyes, without teeth, but generally with a slight notch; anterolateral margins of carapace with 3 or more blunt teeth; abdomen of male with 4 segments plus telson	Family Xanthidae
4b	Anterior margin of carapace, between the eyes, with several distinct teeth; anterolateral margins of carapace with at least 5 teeth; abdomen of male with 6 segments plus telson	5
5a	Carapace more or less five-angled; marginal teeth with several spines; setae stiff, club-shaped	Family Atelecyclidae: Telmessus cheiragonus
5b	Carapace broadly oval; each marginal tooth with not more than 1 spine; setae, when present, not club-shaped.	6
6a	Three teeth between the eyes, 5 anterolateral  carapace teeth	Family Portunidae: , Carcinus maenas
6b	5 carapace teeth between the eyes, 6 or more anterolateral  carapace teeth	Family Cancridae
7a	Carapace almost rectangular, with distinct teeth on the lateral margins; free-living	Family Grapsidae
7b	Carapace oval or nearly circular, without teeth on the lateral margins; closely associated with other invertebrates (living in worm tubes, ascidians, mantle cavity of bivalve molluscs, etc.)	Family Pinnotheridae