392 MONOGRAPH OF THE AUSTRALIAN MARSIPOBRAACHII, 



In connection with the reinstatement of Gray's Velasia I wish 

 to call the attention of those who may ha\ e the opportunity of 

 examining this genus and Geotria during the ammoccetal stage 

 and immediately after the metamorphosis has taken place, to the 

 significance of the dental furrows in the latter genus; from the 

 examination of the adult it appears to me that the evolution of 

 the laminge in Geotria will prove to be materially different from 

 that which holds good for Velasia. 



Finally, it is hoped that the present paper will not only throw 

 some light on the affinities of these various forms, l)ut also induce 

 some of our southern naturalists to spare time for the study of 

 these interesting animals, of whose life history much still remains 

 to be learnt. 



Class MARSIPOBRANCHII. 



The Myzons. 



Skeleton membrano-cartilaginous; skull imperfectly developed, 

 not separate from the vertebral column, which consists of a stout 

 notochord enveloped in a fibrous sheath; neural cartilages present, 

 small; hfemal sheath present in the caudal region only. Lower 

 jaw, ribs, limbs, shoulder-girdle, and pelvic elements wanting. 

 Gills six or more on each side, represented by fixed sacs and 

 destitute of branchial arches. Mouth suctorial and subinferior, 

 more or less circular, with or without lips. Nasal aperture single. 

 Eyes present or absent. Vertical fins present, usually continuous 

 around the tail, supported by feeble rays, which are rarely 

 articulated or branched; no paired fins. Skin naked. Heart 

 without arterial bulb. Air-vessel absent. Alimentary canal 

 straight, little dilated, without pyloric appendages, pancreas, or 

 spleen. Generative openings peritoneal. 



Etymology : — fiapcriTnov, a pouch; jipdyxia, gills; in allusion 

 to the sac-like formation of these organs. 



Distribution : — Seas and rivers of the temperate regions 

 of both hemispheres, no species having as yet been discovered 

 either in high polar latitudes or within the tropics. 



Geologically the Cyclostomes date back to the Lower Devonian. 



