60 CLASS XIV. 



apertures. On the anatomy, the memoir of H. Rathke {Bemerkungen ilber 

 den innern Bau der Priche, Pctromyzon fluviatills. Dantzig, 1835, 4to) may 

 be regarded as the chief work. 



The Dutch Society of Sciences has pubhshed a valuable memoir of 

 Prof. Max Schultze of Halle, Die Entwickelmigsgesch. von Petromyzon 

 Planeri. Mit VIII. Taf. {Natuiirkundige VerJiandelingen van de Hollandsche 

 Maatschappij der Weitensch. te Haarlem. Tiveede Versameling, Deel XII, 

 1856, 4to.) The cleavage of the yolk is entire, and in the first stages of 

 development there is much analogy with that of the frog. 



Ammoccetes Dumeb. Teeth none. Brancliise open internally in 

 the pharynx. External branchial apertures small, placed in a longi- 

 tudinal furrow. Upper lip semicircular. 



Comp. DuMEEiL Dissertation sur les Poissons cyclostomes [Magasin ency- 

 clopedique, 1808); Eathke Beitrdye zur Gesch. der Thier^velt, iv. 1827, s. 

 66 — 102, Tab. II. III. 

 Sp. Ammoccetes hranchialis, Petromyzon hranchialis L., Gu^rin Iconogr., 

 Poiss. PI. 70, fig. 3, Yarrell ^rt<. Fishes, 11. p. 459. In fresh water in 

 many countries of Europe, keeping mostly at the bottom or in the sand ; 

 this .species attains a length of 6 or 7". 



Aug. Mueller {Ueher die Entwickelung der Neunaugen, Mueller's 

 Archil', 1856, pp. 323 — 339) has found that Ammoccetes is the larval form 

 of Petromyzon. The perfect state is not attained until the fourth year from 

 the egg. 



II. Organ of smell double. 



A. Muscular bulb at the base of branchial artery, with nu- 

 merous valves disposed in longitudinal rows. 



Section II. CJiondro])terygiu 



Fins supported by cartilaginous rays. Pectoral and ventral 

 fins. Skeleton cartilaginous; cartilaginous arches closed, forming 

 a canal for the spinal cord; bodies of vertebrae distinct in most, in 

 some the cJiorda dorsalis persistent, situated under the arches in- 

 closing the spinal cord, continuous. Tail recurved upwards, with 

 caudal fin inferior. Skin mostly rough with small bony scales, or 

 covered with large dispersed scutes, sometimes naked. Mouth 

 situated under tlie head. (Ribs distinct in most; swimming-blad- 

 der none.) 



Order III. DesmiobrancMi s. Plagiostomi. 



Brancliiaj adhering to the skin by their outer margin, patent by 

 lateral external apertures on each side. True operculum none. 



