In spiny ant-eaters posterior part of tongue is beset with horny serrations which grind the food against corresponding ridges on the palate. Spiny ant-eaters have a tiny round mouth at the end of long beak. In young platypus flattened, rigid teeth are present and these are replaced by horny structures, formed by an in growth of epidermis beneath them and used for breaking the shells of molluscs. In spiny ant-eaters, teeth are absent in all stages of development.
In platypus the astragalus and calcaneum are firmly united and in male there is an accessory ossification on the inner side for the tarsal spur. The fibula has at its proximal end a compressed process like olecranon process of ulna. There are a radial and two large palmar sesamoid, which are sometimes united.įemur with expanded extremities and with external and internal trochanters. In the carpus the scaphoid and lunar are united and no separate centrale. Fibula is expanded at its upper end like the ulna of other mammals. Humerus with greatly expanded extremities and prominent turbidities and condyles. With the anterior border of pubes are articulated a pair of large epipubic or marsupial bones, presumably for the support of the marsupial pouch. In pelvic girdle pubes and ischia form a long symphysis. The scapula is unlike that of other mammals and with well-developed acromian process and without spine. Coracoid region has two separate bones, coracoid and precoracoid. In the pectoral girdle there are separate clavicles and a median T-shaped interclavicle. Sternal ribs are ossified and connected with vertebral ribs by imperfectly ossified intermediate ribs.
Xiphisternum only in spiny ant-eaters and not in platypus. It consists of a presternum and three keeled stembrae. Ribs articulate with the bodies of the vertebrae and not with transverse processes (i.e., single-headed – only capitulum). In platypus tail caudal vertebrae are twenty or twenty one, each with an inferior spinous process. Sacrum has three or four united vertebrae in spiny ant-enters, and two in platypus, caudal vertebrae in spiny ant-eaters depressed, with no inferior spines but with about five sub-vertebral bones in the form of flat nodules. Thoraco-lumbar vertebrae are nineteen with short transverse processes. Cervical vertebrae are normal and zygapophyses are absent. The epiphyses of the vertebrae are not well developed in platypus and absent in spiny ant-eaters. Quadrate, articular and tympanic have entered the ear. Ossicles in spiny ant-eaters are fused, and separate in platypus. Mandible stouter in platypus and meet anteriorly a short distance and then again diverge and narrow styliform in spiny ant-eaters and loosely united at the symphysis. Small prefrontal and postfrontal bones are present. Tympanic is not united with the periotic. The nasal and premaxillaiy are drawn out into rostrum bearing external nares in both. Postorbital process formed by jugal is present in platypus and absent in spiny ant-eaters. Zygomatic arch is stouter in platypus than that of spiny ant-eaters. Temporal canal perforates the posterior part of zygomatic arch, absent in higher mammals.
Parasphenoid is also present like lower vertebrates. Unlike higher mammals, pterygoid bones are separate. Brain case is larger and much more rounded in spiny ant-eaters than in platypus. The bones of skull early become fused together showing no trace of their boundaries.
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