MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 159 



backwai-dly pointing projection upon its inner end. (Fig. iS.) The bases of the teeth form 

 variou^i angles with the axis of the radula. The outer tooth maizes an angle of about 45 degrees 

 with this axis, while the bases of the third and tifth lateral teeth make angles of 30 degrees with 

 it. The bases of the other teeth are transverse to the axis of the radula. 



The formation of the radula takes place in the extreme posterior part of the ra(Uila sac. 

 The first 8 or 10 rows of teeth are colorless and soft; from here on the teeth gradually turn to 

 dark brown, become hard, and assume their full size. 



From the floor of the mouth a broad, flat process, 12 millimeters in length and 10 millimeters 

 in height, arises at each side of the posterior part of the tongue. The inner sides of these proc- 

 esses bear papilla? to a varying extent in different individuals. They may be entirely covered 

 except for a small space in the center, or only the upper half may be covered. A small, slightly 

 elevated area in the center of the inner side of the process is always free from papilla, and here 

 is a minute pore, the opening of the salivary gland. The salivary glands are ovoid bodies, 

 indistinctly divided into lobules, 5 millimeters by 4 millimeters by 'i^ millimeters, situated within 

 these processes, with their longest axes directed downward and backward. The duct is near the 

 anterior end of the gland; the duct, however, is so short that it scarcely deserves the name, for 

 the glands lie close against the integument of the processes. (Fig. 70.) 



The jaws of the Nautilus are admirably adapted for crushing and biting hard objects. The 

 calcareous layer which covers the inner and outer surfaces of the chitinous parts of both jaws 

 forms a thick, square edge which is practicably unbreakable; slight projecting roughnesses of 

 the edges prevent their slipping upon large and smooth objects. After the jaws have become 

 thoroughly dry the calcareous substance is very brittle and readily peels off the chitinous base, 

 so that in a few days, even without handling, the calcareous matter may have fallen away com- 

 pletely. But while the calcareous matter of the jaws contains any moisture it is extremely tough 

 and hard to remove. This it must always be when the Nautilus is under natural conditions, so 

 there is no doubt but that the calcareous material of the jaws means added strength in no small 



degree. 



Without the addition of the calcareous matter the chitinous parts of the jaws would })e unable 

 to handle the food they do now. In other words, the addition of calcareous material to the jaws 

 is a modification of the typo of strui^ture observal)le in the jaws of other cephalopoda which 

 enables the Nautilus to subsist upon animals possessing shells heavy enough to protect them from 

 most other predatory animals. 



The upper jaw fits snugly inside the lower, so that its outer edge just passes the inner edge 

 of the lower jaw in closing. Consequently when the animal is biting any substance the action of 

 the jaws is not that of a pair of nippers, in which the two jaws meet each other, pinching the 

 substance- in two between them, but instead like that of a steel shear, where two heavy blades 

 having broad flat faces and sharp squai-e corners move past each other. By this construction 

 and motion the maximum I'utting power is attained with the least exertion and the least risk of 

 breaking the edges of the jaws. 



The traps in which the Nautili were caught were baited with chicken, so I often have 

 found the crop filled with large pieces of chicken flesh. Feathers and flesh and bones, even 

 the leg bones, are cut as cleanly as with a pair of shears; everything attests the power of the 

 jaws and their muscles. Apparently also the Nautilus does not pluck its chickens. The 

 remains of some crustacean found in the stomach and intestine are evidence of the ability of 

 the Nautilus to handle any food of this nature. It also seems scarcely doubtful that the 

 Nautilus could eat many of the thinner shelled mollusca, although there is no evidence of 

 such a diet. 



For an animal provided with such powerful jaws a radula seems much like a superfluity. 

 Certainly it can not, as in many of the gastropoda, be the chief organ for seizing and tearing 

 food. The slender and almost delicate form of many of the radular teeth precludes the idea 

 that the radula may be used to rasp away objects which are too hard to be broken by the jaws. 

 It seems possible that the radula may be used to scrape the flesh out of shells, crustacean or 

 molluscan, which have been broken by the jaws. But the contents of the digestive tract 



