UNIVALVE MOLLUSCS — TOOTH-SHELLS — BIVALVES 381 



the group to trie keyhole limpets (Fissurellidce), and so with the rare and beautiful 

 JPleurotomaria, the representatives of which have top-shaped shells with a slit 

 running for some distance along the basal whorl. These shells are of extreme 

 beauty, being marked with scarlet streaks and blotches on a cream-coloured ground, 

 and sometimes growing to a large size. They are of great interest, on account 

 of being, like the New Zealand tuatera and the Queensland lung-fish, survivors of 

 an ancient and once numerous group now on the verge of extinction, allied extinct 

 forms being abundant in the Jurassic and Cretaceous rocks of Europe. 



Here may be mentioned the chiton-shells Chitonidce, of which the flat shells 

 are unmistakable on account of being formed of a number of parallel, transverse, 

 movable plates, so arranged as to permit the molluscs to roll themselves into a ball. In 

 this case, too, the largest species are tropical. Some of these tropical chitons possess 

 eyes on the dorsal surface of the shell, while all have a canal-system within the 

 shell itself which is likewise sensory in function. The dorsal eyes are of two 

 types, one characteristic of the subfamilies Toniciince and Liolojohurince, and the 

 other restricted to certain species of Chiton itself. It is remarkable that in certain 

 species, especially Tonicia chiloensis, the dorsal eyes are attacked, and apparently 

 destroyed, by an alga, which develops within the substance of the shell. 



The tooth-shells (Dentaliidce) constitute by themselves a primary 



group of molluscs, the Scaphopoda, characterised by the rudimentary 

 head, the long foot, and the tubular shell, open at both ends. They are exclusively 

 marine, and generally live with the hind end projecting from the soft sand or 

 mud of the sea-bottom, near the coast. One of the largest species, Dentaliwm 

 elephantinum, of the Mediterranean, Red Sea, and Indian Ocean, has a shell of 

 more than two inches in length, but in the common European elephant's-tooth 

 (D. entale) the shell is half an inch or more shorter, and cream-coloured and smooth, 

 instead of green and fluted. One of the species, D. indianorum, raked up from 

 the sand below low-water mark on the coast of north-western America, was used 

 as currency by the Indian tribes until the Hudson's Bay Company substituted 

 blankets in its place. 



Among bivalved molluscs an interesting genus is Solenomya, 



in which the shell in shape resembles the familiar razor-shells, and 

 has a strong horny outer skin. The soft-parts are characterised by the very long 

 tentacles, and the elongated foot terminating in a circular disc. The genus belongs 

 to a group in which the gill-filaments are more or less inclined at a right angle. 

 To a second group, in which these same gill-filaments are parallel, belong the 

 ark-shells (Arcidce), recognisable by the straight hinges of the two shells, with 

 numerous fine, plate-like, interlocking teeth. Ark-shells are almost cosmopolitan 

 in range, although most numerous in the warmer seas. Strange to say, one species, 

 Area scaphula, inhabits the bed of the Jumna, a thousand miles above Calcutta. 

 Closely allied is the genus Cucullcea, interesting on account of the fact that while 

 there are only two living species, more than two hundred are found fossil ; the 

 present representatives are confined to the coasts of Mauritius, the Nicobars, and 

 southern China. Much the same may be said in the case of the family Trigoniidce, 

 which was believed to be extinct until a specimen of Trigonia pectinata, the 

 one living species, was dredged up in Sydney Harbour. That species exists nearly 



