[Suborder Tetragonuroidei]—Squaretails

Some authors, including Haedrich, include the family Tetragonuridae, the only family of the suborder isolated by L. S. Berg, in the suborder Stromateoidei. An important distinguishing feature of Tetragonuridae noted by Haedrich is the absence of tubules in scales of lateral line; however, the authors observed them in their specimen, even though reduced and partly concealed by imbricate scales. Caudal peduncle thickened, triangular in cross section, with two keels on each side in posterior part formed by special scales. Base of first dorsal fin almost equal to or longer than base of second dorsal fin. Lateral line with very gentle arch in anterior part of body, then continues along middle of body sides and caudal peduncle. In this respect the behavior of Tetragonurus is similar to that of young fish of the suborder Stromateoidei. Presumably Tetragonurus feed almost exclusively on coelenterates and ctenophores; the large knife-shaped teeth on lower jaw and the structure of the mouth are adapted to such food.