34 MOLLUSCOUS ANIMALS. 



into a contrivance for carrying the ova. All this may indicate no more- 

 than a slight analogy, but it is curious and interesting. The best known 

 animal of this order is the Carinaria whose delicate Argonanta-like 

 shell is often seen in collections, and which has been often figured 

 showing its habit of swimming with the fin, formed from the foot up- 

 wards and the back downwards. 



The second order Pulmobranchiata has little direct affinity with the 

 first, but takes this position from an idea that air-breathing marks more 

 active life than water breathing, and from the certainty that the great 

 order Pectinibranchiata occupies among the orders, the position which 

 the class Gasteropoda takes among Mollusca, and the Molluscous sub- 

 kino-dom in the whole animal kingdom ; whilst the lower place of the 

 remaining two orders seems hardly liable to doubt. The most numerous 

 members of the order, the snails and their allies, have the sexes united,, 

 which, considered alone would place them below a large portion of the 

 Pectinibranchiata, but the highest Pulmobranchiata have the sexes 

 distinct and closely resemble the highest vegetable feeders among 

 Pectinibranchiata, the Odontophore corresponding exactly. The present 

 order presents a very natural series and the families are well established. 

 I cannot indeed believe Oncidium the type of a family distinct from 

 Limacidae, or separate Aciculidae from Cyclostomidae. With these 

 reductions the families are : 



1. Cyclostomidae. 



2. Auriculidae. 



3. Helicidae, snails. 



4. Limacidae, slugs. 



5. Lymnaeidae, water snails. 

 Cyclostomidae have a horny operculum, generally a circular mouth 



and an odontophore like that of Litorinidae, with the sexes distinct. 



Auriculidae have no operculum, the aperture elongated and denticu- 

 lated; animal with two tentacles and sessile eyes behind them; a broad 

 muzzle, united sexes, a horny buccal plate, and numerous teeth with a 

 distinct median series. 



Helicidae have usually a well developed shell capable of containing 

 the animal, the body is spiral distinct from the foot. There are almost 

 always 4 tentacles, the upper pair bearing the eyes. There is a horny 

 crescent-shaped buccal plate, the differences of which form valuable 

 characters. The Odontophore is oblong with numerous similar teeth 

 like a pavement. The number of species is very great, and the difi'er- 

 eaces of form striking. 



