PISCES. 59 



Myxine L. Two spiracles approximate on the ventral surface 

 behind the branchiae, each spiracle receiving the external ducts of the 

 six branchiae of its own side. Eyes very small, hidden by muscles. 



Sp. Myxine glutinosa L., Gastrobranchus ccecus BLOCK, LINN. Mus. Ad. Fred. 

 Tab. vin. fig. 4, BLOCK Syst. Ichth. Tab. 104; CUVIEB R. Ani., ed. itt., 

 Poiss. PL 1 20, fig. 3 ; lives in the North Sea. LINN.EUS referred it to the 

 worms, regarding the two lateral parts of the tongue as transverse jaws 

 (maxillce transver sales), which do not occur in vertebrate animals. There is 

 a duct which leads immediately from the oesophagus to the left spiraculum ; 

 a similar duct occurs also in the following genus, going to the posterior 

 spiraculum branchiate of the left side. The animal is about 1 1" long. 



Bdellostoma MUELL., Heptatrema DUMER. Branchiae on each 

 side six or seven, each supplied with an external spiracle. Eyes 

 small, conspicuous through the skin. 



Fishes from the southern seas, for which the name Heptatrema can 

 scarcely be retained, since individuals are met with having six gill-apertures 

 on each side, or six on the right side and seven on the left ; the species are 

 not yet sufficiently defined ; they closely resemble Myxine glutinosa in 

 internal structure, notwithstanding the external characters ; frequently, how- 

 ever, they attain a larger size. There are seven gill-apertures on each side 

 in Petromyzon cirratus, Bdellostoma Forsteri MUELL. ; see BLOCK Syst. Ichth. 

 pp. 531, 533. To this genus belongs a species imperfectly described and 

 badly figured by LACEPEDE Gastrobranchus Dombey; Poiss. I. PL 23, fig.i. 



Family III. Petromyzonini (Hyperoartia) . Body cylindrical, 

 with mouth anterior, lip circular or lunate. Olfactory cavity open 

 above by an external nasal foramen, produced posteriorly into a 

 blind canal, not perforating the palate. Thorax cartilaginous, sus- 

 taining the branchial apparatus, composed of strips descending on 

 each side beneath the skin from the back towards the ventral sur- 

 face. Two dorsal fins, the posterior conjoined with the caudal fin. 



Petromyzon L. (exclusive oiPetr. Iranchialis} . Several labial and 

 lingual teeth. Branchiae open internally in a suboesophagean tube, 

 blind posteriorly. Labial ring circular, margined by many small cirri. 



Sp. Petromyzon marinus L., BLOCK Ichth. Tab. 77, GUERIN Iconogr., Poiss. 

 PL 70, fig. i, YAERELL Brit. Fishes, n. p. 448 ; Petromyzon fliiviatilis L., 

 BLOCK Ichth. Tab. 78, fig. T, Cuv. R. Ani., ed. iU., Poiss. PL 120, fig. i 

 (named Petr. marin.). The first of these species, the marine, comes on the 

 approach of summer into rivers, and attains a length of more than two feet ; 

 the last, much smaller, appears to live constantly in fresh water. This 

 species is named Lamprey (Lampetra from lambendo petras), Pricke, &c. 

 These animals use their tongue as a sucker, and attach themselves to 

 different bodies ; they then inspire and expire by the external branchial 



