64 COMPARATIVE ZOOLOGY. 
are the convex edges of horny semicircles, each bordered 
by a row of nearly a hundred hard, sharp teeth. When 
the mouth, or suck- 
er, is applied to 
the skin, a sawing 
movement is given 
to the horny ridges, 
so that the “bite” 
Fig. 27.—Teeth and Masticatory Apparatus of Gastero- c 
pods: A, portion of odontophore, or “ tongue,” of Vel- of the Leech is re- 
utina, enlarged; B, portion of odontophore of Whelk 
(Buccinum undatum), magnified —the entire tongue 
ally a saw-cut. 
has 100 rows of teeth ; C, head and odontophore of Lim- The dentition of 
pet (Patella vulgata); D, portion of same, greatly mag- : 
nified, to show the transverse rows of siliceous teeth. the univalve Mol- 
lusks, or the Snails, is general- 
ly lingual, 2. ¢., it consists of 
microscopic teeth, usually si- 
liceous and amber- colored, 
planted in rows on the tongue. 
The teeth are, in fact, the ser- 
rated edges of minute plates. 
The number of these plates 
varies greatly; the garden 
Slug has 160 rows, with 180 
teeth in each row. 
All Birds, and some other 
Vertebrates, as Ant - eaters,” 
Turtles, Tortoises, Toads, and 
Sturgeons, have no teeth. 
Their absence is generally 
associated with a horny beak, 
a wide gullet, and a muscular 
stomach (gizzard). 
In a few Vertebrates, horny 
plates take the place of teeth, 
as the Duck Mole (Ornitho- 
rhynchus) and Whalebone 
Fra. 28.—Section of the Upper Jaw of 4 
Whale (Balenoptera), showing baleen- 
plates: a, superior maxillary bone; 8, 
ligamentous gum attaching the horny 
body of the baleen-plate, c; d, fringe 
of bristles; e, smaller plates, 
