FISHES. 
313 
SUB-CLASS II.—OSSEOUS FISHES. 
It consists of six orders:— 
IV. Plectognathi.— V. Lophobranchii.— VI. Mala- 
COPTERYGII AbDOMINALES.- VII. MaLACOPTERYGII SuBRA- 
CHIATI.— VIII. MALACOPTERYGII ApODES.— IX. ACANTHOP- 
terygii. 
SUB-CLASS I.-CARTILAGINOUS FISHES. 
ORDER I.—CYCLOSTOMI. 
Jaws fixed in an immoveable ring; branchiae adhering, with 
numerous openings. 
Genus 1.—PETROMYZON.— Dumeril. 
Generic Character. —Maxillary ring provided with strong 
teeth ; lips formed for sucking ; tongue beset with two rows of 
minute teeth; a dorsal fin before, and another behind the anal 
region. 
Petromyzonfiuviatilis. — The Lesser Lamprey. 
Plate LXXIV. fig. 1. 
Back dark olive-green ; silvery-white beneath; first and se¬ 
cond dorsal fins distinct; top of the maxillary ring provided 
with two large teeth. Eighteen inches long. Inhabits the 
rivers in Britain. 
Genus 2. — AMMOCCETES.-— Dumeril. 
Generic Character. —Body membranous, soft; openings of 
the branchiae similar to those of the lamprey’s; lip luniform, 
covering the upper part of the mouth only; destitute of teeth ; 
dorsal and caudal fins united, and forming a sinous ridge. 
Genus 3—GASTROBRANCHUS.— Bloch. 
Generic Character.— Maxillary with a single tooth above, 
and membranous; tongue with strong lateral dentations, and ar¬ 
ranged in double rows on each side; two spiracles situated un¬ 
der the abdomen. 
