ANCEIDA. 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 (Praniza), 
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 
Anceide. Strictly speaking, however, the name Gnat- 
hide 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. 
