Sentence:  The <WSD>chimeras</WSD> ... are an extant group of about 30 species. They have the upper jaw fused with the cranium and a gill cover over the four gill slits. They also have toothy plates that give them a ratlike appearance, thus the common name "ratfish." The group occurs in ocean depths worldwide, where they mainly feed on invertebrates. 
Meaning of the word marked with <WSD> tags:  "Usually" '''(chimaera)''': a cartilaginous marine fish in the subclass Holocephali and especially the order Chimaeriformes, with a blunt snout, long tail, and a spine before the first dorsal fin.