HOMALOGYRA. 



67 



Genus II. HO'MALOGY'RA* (formerly Omalogyrd), 

 Jeffreys. PL I. f. 5. 



BODY flattened: tentacles wanting: eyes quite sessile, and 

 placed behind the head. 



SHELL forming a flat coil, and having an involute spire : 

 u'horls more or less angulated : mouth clasping both sides of 

 the periphery : operculum few-whorled, with a central nucleus. 



The animal is unlike that of any known Pectinibran- 

 chiate mollusk ; and, if we except Omalaxis or Bifrontia, 

 the shell has no existing parallel among the marine uni- 

 valves. In the latter respect it may be regarded as the 

 analogue of Planorbis. The upper part of the body of 

 H. atomus is partially ciliated. This character is exhi- 

 bited to a greater extent in Stilifer, as well as in the 

 tentacles of Trochus, Rissoa, C&cumj and other genera. 

 Dr. Fischer was rather too positive in stating (Journ. 

 Conch, vii. p. 365) that my observations proved, " d^une 

 maniere irrefragable/' the animal of the present genus 

 to be the fry of some mollusk. The only instance ad- 

 duced by him in support of such a conjecture is the 

 change which many of the Nudibranchs undergo in the 

 larval state. Their embryonic shells, however, have a 

 rudimentary spire of scarcely a single whorl, and are 

 all of the same size in each species, the animals are 

 natatory, and the metamorphosis is of short duration. 

 The shells of Homalogyra, on the contrary, have a com- 

 plete spire of from 3 to 4 whorls, and are of various sizes 

 (indicating different stages of growth) ; the animals 

 crawl about, and they are met with at all seasons of 

 the year. There is no more reason to suppose that 

 Homalogyra is an immature mollusk than Skenea, Cy- 

 clostrema, or any other minute kind. The tongue of 



* A flat circle. 



