The exoskeleton consists principally of calcifications forming dorsal and ventral shields which cover the head and abdominal region; the dorsal shield is formed of a few plates firmly united, the ventral shield of a single plate. The shields are composed of three layers, the middle layer being traversed by canals belonging to the dermal sense organs which open to the exterior by a series of pores. The tail is sometimes covered by scales. The orbits are widely separated and laterally placed. Paired appendages are absent. These curious forms are found in beds of Upper Silurian and Lower Devonian age. One of the best known genera is _Pteraspis_. _Suborder_ (2). OSTEOSTRACI. The exoskeleton as in the Heterostraci consists of shields and scales, the shields being divisible into three layers. The anterior part of the body is covered dorsally by a single large shield which differs from those of the Heterostraci in having the inner layer ossified. The middle layer contains canals for the passage of blood vessels, but the exoskeleton shows no impressions of dermal sense organs. The posterior part of the body is covered by large quadrangular scales. Paired appendages are absent, but median dorsal and caudal fins occur supported by scales, not fin-rays. _Cephalaspis_, the best known of these animals, occurs in beds of Lower Devonian age. _Suborder_ (3). ANTIARCHA. The exoskeleton is formed of bony plates, the dorsal and ventral shields each consisting of several symmetrically arranged pieces. The tail may be covered with small scales or may be naked. The head is articulated with the trunk, and its angles are drawn out into a pair of segmented paddle-like appendages, covered with dermal plates. The orbits are close together. A dorsal fin and traces of mouth parts occur in _Pterichthys_, but the endoskeleton is unknown. The best known forms _Pterichthys_[26] and _Asterolepis_ occur in beds of Lower Devonian age. GENERAL ACCOUNT OF THE SKELETON OF MARSIPOBRANCHII. The Marsipobranchii are worm-like animals. The living forms include two families, the Myxinoidei (Hags)--genera _Myxine_ and _Bdellostoma_--and the Petromyzontidae (Lampreys). Three species of _Petromyzon_ are known, _P. fluviatilis_, _P. marinus_ and _P. planeri_. The larval forms were for a long time thought to belong to a separate genus and were called _Ammocoetes_. The Myxinoids, although very highly specialised in their own way, are at distinctly a lower stage of development than the adult Lamprey, and come nearer to the larval Lamprey or Ammocoete. SPINAL COLUMN. [Illustration FIG. 4. A, DORSAL; B, LATERAL AND C, VENTRAL VIEW OF THE SKULL OF _Petromyzon marinus_ × 1 (after PARKER).