NOTES. 381 
According to Owen, the only character which absolutely distinguishes 
Fishes and Reptiles is whether or not there is an open passage from the 
nostrils to the mouth. 
7 Some Ichthyologists, as Agassiz, Haeckel, Cope, and Gill, divide the 
Vertebrates below the Amphibians into three or four distinct classes. See 
“Smithsonian Mise. Coll.,’’ vol. xi. 
8 It would be safe to say that any living Vertebrate with side fins sup- 
ported by fin-rays is a Fish; but the extinct Amphibian Jchthyosaurus also 
had them. 
179 The capacity for growing as long as life lasts, which some Fishes are 
said to possess, may be explained by the facts that their bodies are, firstly, 
of very nearly the same specific gravity as the water in which they live, and, 
secondly, of a temperature which is but a very little higher than that which 
they are there exposed to. Thus the force which in other animals is ex- 
pended in the way of opposition to that of gravity and in the way of pro- 
ducing heat is available for sustaining continuous growth.—ROLLESTON. 
180 Scales with smooth, circular outline are called cycloid; those with 
notched or spiny margins are ctenoid. A few Teleosts are without scales or 
osseous skeleton. The ventral fins are often wanting, and the pectoral oc- 
easionally. There are about 9000 species of bony fishes. 
181 Amphibians with a moist skin are also remarkable for their cutaneous 
respiration. They will live many days after the lungs are remoyed. Their 
vertebre vary in form: in the lowest they are biconcave, like those of 
Fishes; in Salamanders they are opisthocelian: in the Frogs and Toads 
they are usually proccelian. 
182 Salamanders are often taken for Lizards, but differ in having gills in 
early life and a naked skin. The Proteus and Siren resemble a tadpole ar- 
rested in its development. 
183 The Surinam Toad has no tongue. 
184 The Reptilian heart may be likened to a persistent foetal heart of the 
higher animals. 
185 The posterior pair of limbs is sometimes represented by a pair of small 
bones; and there is one Ophidian which shows traces of external limbs. 
186 There are some notable exceptions. The Slow-worm is legless, and 
the Chameleon has a soft skin. 
187 According to Owen; but Huxley insists that the plastron belongs to 
the exoskeleton. 
188 Knees always bend forward, and heels always bend backward. 
189 Tt is a peculiarity of all Birds, though not confined to them, that the 
generation products and the refuse of digestion are all discharged through 
one common outlet. 
199 Existing Birds have been divided into two primary groups, accord- 
ing to the development of the breast-bone: (1) Ratite, or Runners, as 
the Ostrich, without a keel; and (2) Carinate, or Fliers, comprising 
the grand majority of Birds, having a prominent keel. The fossil Ar- 
cheopteryz, a lizard-like Bird, is placed in a separate division, Saurure. 
Birds have also been divided according to their degree of development 
at birth into (1) Hesthogenous, as Fowls, Ostriches, Plovers, Snipes, Rails, 
