4/0 NOTES 



the margin of the mantle ; and the bend in the pallia! line, called pallial 

 sinus, which exists in those shells having retractile siphons, as the clam. 



27 The clam is the highest of lamellibranchs, and the oyster one of the 

 lowest. The Mya arenaria, or " soft clam," has its shell always open a 

 little; while Venus mercenaria, or "hard clam," keeps its shell closed 

 when disturbed. 



28 The slug has no shell to speak of. It may be remembered, as a rule, 

 that all univalve shells in and around the United States are gastropods, 

 and that all bivalves in our rivers and lakes, and along our seacoasts (save 

 a few brachiopods), are pelecypods (lamellibranchs). 



29 Hold the shell with the apex up and the mouth toward the observer. 

 If the mouth is on his right, the shell is right-handed or dextral, if on his 

 left, sinistral. In other words, a right handed shell is like a right-handed 

 screw. 



30 Instead of a strong breathing tube with a valve, answering for a force- 

 pump and propeller, as in the cuttlefish, it has only an open gutter made 

 by a fold in the mantle, like the siphons of the gastropods. The back 

 chambers are filled with gas. 



The common poulpe has two thousand suckers, each a wonderful little 

 pump, under the control of the animal's will. 



81 The facial angle becomes of less and less importance as we go away 

 from man, and for two reasons. Where the brain does not fill the brain 

 case the angle is obviously of little value, and if the jaws are largely de- 

 veloped the angle is reduced, although intelligence may not be altered. 



32 Oblong human skulls, whose diameter from the frontal to the occipi- 

 tal greatly exceeds the transverse diameter, are called dolichocephalic ; and 

 such are usually prognathous, i.e., have projecting jaws, as the negro's. 

 Round skulls, whose extreme length does not exceed the extreme breadth 

 by a greater proportion than 100 to 80, are brachycephalic ; and such are 

 generally ortkognathous, or straight-jawed. 



33 The classes are variously grouped into the Hematocrya, or cold- 

 blooded, and the Hematotherma, or warm-blooded ; into the Branchiata 

 and Abranchiata ; into the Allantoidea and Anallantoidea. 



34 Amphibians with a moist skin are also remarkable for their cutaneous 

 respiration. They will live many days after the lungs are removed. Their 

 vertebrae vary in form : in the lowest they are biconcave, like those of 

 fishes ; in salamanders they are opisthocoelous : in the frogs and toads 

 they are usually proccelous. 



85 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 

 arrested in its development. 



86 The Surinam toad has no tongue. 



37 There are some notable exceptions. The slow worm is legless, and 

 the chameleon has a soft skin, with minute scales. 



