AND DISTRIBUTION OF THE FIROLIDÆ. 275 
The position of the visceral nucleus and character of the branchial apparatus further 
permit us to distinguish between the two latter thus :— 
Firola. Body terminated by a tapering tail, visceral muscles situated in front of the 
latter ; respiratory organ consisting of well-developed branchiæ or a ciliated subspiral 
band. : 
Firoloides. Body abruptly truncate posteriorly ; visceral nucleus terminal; respiratory 
apparatus consisting of a very rudimentary ciliated organ, or altogether absent. 
This distinction between Firola and Firoloides, first pointed out by Lesueur, is unques- 
tionably the easiest and the best, and too little attended to by observers. In following 
it, however, they must avoid confounding the tail proper (metapodium) with the singular 
annulated filiform appendage often present when the former is wanting, as in some 
varieties of Firoloides. The presence or absence of the tentacles and the position of the 
fin, on which stress has been laid by some, do not appear characteristic or distinctive of 
species, or available for the purpose of classification. The subjoined Table shows the 
distinguishing zoological marks of the Heteropoda IE to this arrangement. 
Atlanta. —Shell calcareous, spiral ; opereulum 
val, nucleus median. 
4 Attantoides (Oxygyrus).—Shell corneous, in- 
| volute ; operculum trigonal; nucleus la- 
L erg 
( Visceral nucleus only, pro- Carinaria.—Shell vitreous, spiral. 
| tected by a shell, pedun- | Carinaroides Mii, pe —Shell corneous, 
ATLANTIDÆ.— Body curved and re- 
tractile into spiral shell .... ....................... 
culated, situated at pos- involute, or absen 
terior third or fourth. 
Heteropoda. 
¿A 
Frrozrnæ. — Body elongated and 
\ s Firola.—Body terminated by a tapering tail ; 
naked, or nearly so 
visceral nucleus situated i in front of this ; 
Visceral nucleus shell-less< respiratory apparatus consisting of true 
| and sessile, | 
ucleus terminal ; Bis ione apparatus 
consisting of a rudimentary ciliated 
organ, or entirely Kong 
