Xix., 1871, p. 236, &c.) therefore relied upon more fundamental characters, notably the presence or absence of osteoderms, the formation of the skull, the teeth and the tongue.
Presumably the presence of osteoderms and of complete cranial arches are more archaic than their absence, just as we conclude that limbless forms have been evolved from various groups possessed of fully developed limbs.
- Acrodont; tongue broad and thick, not protractile; no osteoderms. Old World.
- Pleurodont; tongue broad and thick, not protractile; no osteoderms. America, Madagascar and Fiji Islands.
- Pleurodont; solid teeth; anterior part of tongue slightly emarginate and retractile, and covered with flat papillae; no osteoderms. Mexico.
- Pleurodont; teeth solid, sometimes (Ophiosaurus) grooved; anterior part of tongue emarginate and retractile into the posterior portion; osteoderms on the body, and especially on the head where they are roofing over the temporal fossa; entirely zoophagous and ovo-viviparous.
Lanthanotus corneensis, of which only a few specimens are known, is apparently closely allied to Heloderma, although the teeth are not grooved, osteoderms are absent and probably also the poison glands.
- Pleurodont; tongue short, villose, scarcely protractile, feebly nicked at the tip. With osteoderms at least upon the skull, where they roof in the temporal region.
- Pleurodont; tongue very short and scaly; no osteoderms; supratemporal fossa roofed over by the cranial bones; eyes devoid of movable lids; tympanum exposed; femoral pores present; limbs and tail well developed.
- Teeth solid, almost acrodont; tongue long and narrow, deeply bifid, beset with papillae; no osteoderms; scales of the back very small or quite granular; limbs sometimes reduced.
Osteoderms on the head and body.
Similar osteoderms underlie the scales of the body and tail.
The worm-shaped body is devoid of osteoderms. The tongue is short, covered with imbricating papillae and slightly nicked anteriorly.
The vermiform body is covered with cycloid imbricating scales, devoid of osteoderms. Limbs and even their arches are absent, excepting a pair of flaps which represent the hind-limbs in the males.
Osteoderms on the head and body, roofing over the temporal region.
Tongue long and bifid, with papillae or folds, with osteoderms on the head but not on the body.