37 ° 
MOLLUSCS, 
genera it is as large as the aperture of the shell (. Ampullaria , Natica, Bithynid), 
while in others it only partly closes it; and in other cases it is so small as to be of 
no use for closing the shell. It is mostly horny, but in a few instances is more or 
less thickened and strengthened with a shelly or calcareous layer. The horny 
opercula are mostly flattened, but in a few cases are thickened, or conical ( Torinia , 
Codopoma). The shells of Prosobranchs comprise some of the most beautiful, and 
most prized by collectors. The order may be separated into three main divisions, 
namely, Pectinibranchiata, Heteropoda or Nucleobranchiata, and Scutibranchiata, 
severally characterised by differences, as the names imply, in the breathing-organ, 
coupled with other characters in connection with the genital organs, radula, etc. 
Suborder Pectinibranchiata. 
The molluscs included in this suborder have the body twisted, and contained 
in a spirally-coiled shell. They are chiefly aquatic, and mostly marine, but a few 
families are strictly terrestrial. The gills are comb-like, that is, consist of plates 
arranged side by side and attached to a common stem, like the teeth of a comb to 
the thickened back. They are situated in a vaulted chamber, formed by the mantle 
over the neck, which is open in front for the admission of air or water. The 
Pectinibranchs have been subdivided into several sections, founded upon modifica¬ 
tions of the radula or lingual ribbon. The following are the most important in 
the arrangement adopted by Paul Fischer, namely, Toxoglossa, Rhachiglossa, 
Trenioglossa, Ptenoglossa, and Gymnoglossa. A large proportion of the Pectini¬ 
branchs are furnished with a long proboscis, extensile and retractile at the will 
of the animal. In almost every instance, molluscs of this proboscis-bearing group 
are provided with shells which may be recognised by a notch or canal at the front 
of the aperture, through which the so-called siphon is protruded. This consists of 
a more or less elongate fold of the mantle, and conveys the water to the branchiae. 
In another large section of the Pectinibranchs—Rostrifera—there is no extensile 
proboscis," but the head terminates in a more or less prolonged snout which is not 
retractile, and is termed the rostrum. These two groups, however, are now 
abandoned on the ground that better characters are furnished by the radula. 
SECTION TOXOGLOSSA. 
In this group, including the families Conidae, Terebridce, Pleurotomidce, and 
Cancellariidce, the radula is composed of two rows of long, barbed, marginal teeth. 
The cones. ( Conidce) form an extensive family, containing about five hundred and 
fifty species. They have always been great favourites with collectors, on account 
of the great beauty and variety of the colour-markings, and are almost exclusively 
tropical, only a few species ranging northward to the Mediterranean and Japan. 
Some of them have a very wide distribution, being found in the Red Sea, Indian 
Ocean, and in various parts of the Pacific. Others, on the contrary, have an 
equally limited range. These animals are all predaceous, having a short strong 
foot, truncate or square in front, and pierced with a water-bearing pore at the 
anterior part. The head has two slender tentacles, with the eyes on the outside 
