Branchiae purse-shaped, fixed, opening outwards by several apertures: jaws represented by an immoveable cartilaginous ring, formed by the union of the palatine and mandibular bones: no pectorals or ventrals: body elongated: the skeleton very imperfectly developed.

77. Petromyzon

Seven branchial openings on each side: maxillary ring armed with strong teeth; inside of the mouth furnished with tooth-like tubercles: lip circular: tongue with two longitudinal rows of small teeth.

78. Ammoccetes

Branchial openings seven in number: lip semicircular, and covering only the upper part of the mouth: no teeth; but the opening of the mouth furnished with a row of small branched cirri.

79. Myxine

Branchial openings two in number, placed beneath: maxillary ring entirely membranaceous, with a single tooth above: tongue with strong teeth arranged in two longitudinal rows on each side: mouth circular, surrounded by eight barbules; a. spiracle above, communicating with the interior.