. Outlines of zoology. addle-like flippers, and there are no externalhints of hind-limbs beyond mere button-like knobs in someembryos. Most forms have a median dorsal fin. Hairsare generally absent, though a few bristles may persist nearthe mouth. The thick layer of fat or blubber beneaththe skin retains the warmth of the body, and compensatesfor the absence of hair. In one of the dolphins dermalossicles occur, a fact which has suggested the idea that thetoothed whales may have had mailed ancestors. Tracesof dermal armour have also been found in the extinctZeuglodonts. The spindle shape, the a
. Outlines of zoology. addle-like flippers, and there are no externalhints of hind-limbs beyond mere button-like knobs in someembryos. Most forms have a median dorsal fin. Hairsare generally absent, though a few bristles may persist nearthe mouth. The thick layer of fat or blubber beneaththe skin retains the warmth of the body, and compensatesfor the absence of hair. In one of the dolphins dermalossicles occur, a fact which has suggested the idea that thetoothed whales may have had mailed ancestors. Tracesof dermal armour have also been found in the extinctZeuglodonts. The spindle shape, the absence of external ears, theabsence of an eye-cleansing nictitating membrane, the dorsalposition and valvular aperture of the single or double nostril,the sponginess of the bones, the retia mirabilia storing CETACEA. 767 arterial blood in different parts of the body, may be asso-ciated with the aquatic life. The cervical vertebrae are thin, and more or less is no union of vertebrae to form a sacrum, for the. Fig. 412.—Left fore-limb ofBalanoptera. Scapula with spine (j^.); //.,?humerus; i?.,radius ;i/., ulna ;C, carpals embedded in matrix ;Ale, metacaipals; /^/i., phal-anges. Fig. 413.—Fore-limbof whale (Megapleralongimana). —AfterStruthers. hind-limbs are at most very rudimentary. Under thecaudal vertebrae there are wedge-shaped chevron bones. The brain-case is almost spherical; the supraoccipitalmeets the frontals and shuts out the parietals from the roofof the skull; the frontals arch over the orbit; the snout orrostrum of the skull is composed of premaxilte, maxillse. 768 MAMMALIA and vomer, and of the mesethmoid cartilage. The perioticin whales is an exceedingly dense bone, and is of interestbecause it is the only part of the skeleton found at greatdepths on the floor of the ocean, and is often preserved asa fossil. There are at least rudiments of two sets of teeth, asin other Mammals, but in baleen whales only the teeth ofthe milk set are calcified,
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
