60 CLASS XIV. 



apertures. On the anatomy, the memoir of H. RATHKE (Bemerkunyen iiU'i' 

 den innernSau der PricJce, Petromyzon fluviatilis. Dantzig, 1825, 4to) may 

 be regarded as the chief work. 



The Dutch Society of Sciences has published a valuable memoir of 

 Prof. MAX SCHULTZB of Halle, Die EntwicTcelungsgesch. von Petromyzon 

 Planeri. Mit vui. Taf. (Natuurkundige Verhandelingen van de ffollandsche 

 Maatschappij der Wettensch. te Haarlem. Tweede Versameling, Deel xu, 

 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. Branchiae open internally in 

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

 tudinal furrow. Upper lip semicircular. 



Comp. DUM^RIL Dissertation sur les Poissons cyclostomes (Magasin ency- 

 clopedique, 1808); KATHKE Beitrdge zur Gesch. der ThierweU, iv. 1827, s. 

 66 102, Tab. H. in. 



Sp. Ammoccetes Iranchialis, Petromyzon branchialis L., GURIN Iconogr., 

 Pom. PI. 70, fig. 3, YARRELL Brit. FisJies, n. 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 (Ueber die Entwickelung der Neunaugen, MUELLER'S 

 Archiv, 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. Chondropterygii. 



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 chorda 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 the head. (Bibs distinct in most; swimming-blad- 

 der none.) 



ORDER III. Desmidbranchii s. Plagiostomi. 



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

 lateral external apertures on each side. True operculum none. 



