ANCEID^E. 169 



middle tail-piece (telson), a five-lobed organ useful in 

 swimming to guide the animal. 



The animals composing this most anomalous family 

 have, until quite recently, been considered as forming, 

 not only two distinct genera, but even have been ar- 

 ranged by M. Edwards and Dana as distinct tribes. The 

 observations of M. Hesse, of Brest, however, leave no 

 room to doubt that the two supposed genera, Anceus and 

 Praniza, are but the males and females of one genus, 

 although his assertion that they are but different 

 phases of one and the same animal cannot be ac- 

 cepted. It seems, however, sufficiently proved, that 

 one of these animals of small size, which would hereto- 

 fore have been unhesitatingly called a Praniza, if kept in 

 confinement, becomes developed into a female (Pranizci), 

 while a second, only slightly differing, becomes a fully 

 developed male (Anceus). We therefore adopt the sug- 

 gestion of M. Hesse (not indeed for the reason which he 

 assigns, that a Praniza is transformed into an Anceus 

 a statement which our observations will sufficiently dis- 

 prove), but in conformity with the usual rule of priority, 

 as well as that of accepting the denomination of the 

 male rather than that of the female animal, and, ac- 

 cordingly, retain for the group the family name of 

 Anceida. Strictly speaking, however, the name Gnat- 

 hiida should be given to the family, Leach's name 

 Gnathia having the priority in point of date over that of 

 Anceus; but as Leach himself omitted the genus in his 

 later works, and there is a well acknowledged genus, 

 Gnathium, amongst the heteromerous beetles proposed by 

 Kirby, we have preferred using the generally adopted 

 name of Anceus. 



