THESE minute colonial organisms leave skeletons which may be found among the washings of clays and sands, but which may otherwise be often overlooked. Many of the colonies, however, attain to a considerable size.

The hard parts are built up of an external aragonite layer and an internal calcite layer (Cornish and Kendall).

Almost all are marine.

Terms used :

:

Polypide or Zooid.-The individual animal.

Zoarium. The colonial structure formed by the polypides. Commonly attached or encrusting.

Zoæcium or Cellule.-The tube-like or ovoid chamber occupied by each polypide.

Ovicell.-A chamber for containing one or more eggs, from which embryo-polypides develope and are set free. The ovicell forms a swelling above the aperture of certain zoœcia in cheilostomatous colonies, and an inflation between the zoœcia in cyclostomatous colonies.

Operculum. The cover that closes the aperture of the zoœcium in some polyzoa.

Avicularia and Vibracula.-Beak-like and whip-like appendages respectively, set on stalks and arising from little special pits on or between the zoœcia (fig. 58). Used in obtaining food, or for defensive purposes. They are in reality specially modified

zoœcia.

A. CYCLOSTOMATA.

Zoœcia tubular, typically not narrowing towards the aperture; no operculum. Calcareous (aragonite with some calcite, Sorby); rarely horny.

Entalophora.-Zoarium branching. Zoccia in the form of long curving tubes, which open all round the surface of the twiglike zoarium. Marine.

Gotlandian to Recent.

Fascicularia.-Zoarium spheroidal, fixed at base. Zoccia tubular, often curving, united into bundles which radiate from the base, leaving hollow interspaces. Marine.

Pliocene.

B. CRYPTOSTOMATA.

The Fenestellida here form the most important family, in which the zoœcia show considerable deviation from the tubular type; in section, they are seen to be more complex and narrowed at the external aperture. The latter is, however, round and simple, as in the typical cyclostomata.

Fenestella (figs. 56 and 57).-Zoarium lamellar, the sheet-like mass being commonly folded into the shape of a funnel, often

several inches across.

Built up of rods which radiate from the base and are connected by little cross-bars so as to form a network. The minute zoœcia are grouped in two rows on each of these rods. Sometimes a third central row occurs. The zoœcia must be looked for with a lens ; and the far larger interspaces of the mesh are styled fenestrules. On the systematic position of this genus see Ulrich, Geol. Surv. of Illinois, vol. viii., p. 349. Marine. Gotlandian to Permian; particularly Carboniferous.

C. CHEILOSTOMATA.

Zoccia typically ovoid, not tubular; the aperture is in the side and near the upper end, and is smaller than the diameter of the zoœcium.

This aperture was closed by an operculum in most forms. The pits occupied by avicularia and vibracula can often be recognised. Horny or calcareous (aragonite, with some calcite?).

Eschara (fig. 58).-Zoarium formed of two layers of zoccia, back to back, producing a sheet-like mass which branches as

it spreads. Zoccia close-set, typically cheilostomatous, with pits where the appendages have fallen away; the zoœcia of one row alternate with those of the next. Marine.

Jurassic to Recent.

Lepralia-Much like Eschara, but forming one encrusting layer. Marine.

Cretaceous to Recent.

Membranipora.-Zoarium encrusting. Zoœcia rather flat, with raised margins, and touching one another along their borders, more commonly than overlapping. The front of each is generally lost, having consisted of a chitinous membrane, a wide shallow cavity being thus revealed. Arrangement of zoœcia rather irregular. Marine.

Jurassic to Recent.

Cellepora.-Zoarium built up of zoœcia piled irregularly on one another, and thus forming a mammillated aggregate fixed at Sometimes branching. Zoccia fairly ovoid. Marine.

the base. Cainozoic.

VI. Brachiopoda.

The Brachiopoda, formerly more prominent than the Mollusca, inhabit a bivalve shell, composed of calcite, or occasionally of phosphate of lime; this is minutely perforated over the whole surface in almost all the families. The Rhynchonellida form an important exception, being imperforate (impunctate). Hollow spines, often of great length, project in some genera from the surface of the shell. The valves of the shell are typically unequal; even if apparently equal, their internal structure is very different. the shell so that the small valve faces the observer, and the umbo of the large valve forms the highest point of the shell; a vertical plane passing through the umbos and the point opposite to them on the lower border divides the shell into two symmetrical halves. Compare Lamellibranchiata. (See fig. 59.)

Hold

The modern species, which are numerous and all marine, mostly inhabit deep water.

The modern classification of the brachiopoda is discussed by Schuchert, Bull. U. S. Geol. Survey, No. 87 (1897), pp. 113-135. For our present purposes, the two large divisions of Articulata and Inarticulata will prove sufficient.

Terms used:

Ventral Valve. The larger valve of the shell; or in any case that covering the ventral portion of the animal.

Dorsal Valve.-The smaller valve, or that covering the dorsal portion.

Both valves are commonly perforated by minute canals, being then said to be punctate (fig. 65).

Both show somewhat oval Muscular impressions, placed below the hinge. In addition, Vascular impressions, indicating the position of blood-vessels in the mantle, are sometimes seen as faint grooves ramifying over the internal surface of both valves; these may appear as ridges on internal casts.

The ventral valve terminates posteriorly in a more or less sharp Beak or Umbo. An aperture, the Foramen, may occur in this, or just below it, serving for the exit of the fibrous pedicle by which the animal was attached. When below the beak, it is generally triangular. In the dorsal valve, the beak is usually less prominent.

Deltidium.-A triangular structure found below, and partly or wholly surrounding, the foramen in many forms. It consists of two little plates, generally meeting along part of their length, and arising from opposite sides below the beak, thus limiting the aperture (fig. 59).

Pseudodeltidium.-A plate formed occasionally across the foramen and spreading anteriorly from the beak.

Area. The flat area often occurring between the hinge and the beak; sometimes striated. Commonly seen in the ventral valve, rarely also in the dorsal. It stretches on either side of the triangular foramen, the deltidium, or the pseudodeltidium.

Teeth. Two processes set in the ventral valve, and commonly borne by two "dental plates," which are like short septa rising from the inner surface of the valve. The teeth occur on the Hinge-line (or line along which the two valves are united during life). They fit into two sockets in the dorsal valve.

The dorsal valve bears ordinarily a Cardinal Process projecting somewhat down from the centre of its hinge-line. To this the muscles that opened the shell were attached. Two plates called Crura, one on each side of the centre of the hinge-line, may occur in this valve, the "arms," or lamellar mouth-appendages of the animal, being then attached to them. In most cases they bear a Brachial Loop, a calcareous structure of great delicacy, which supports the arms. The loop sometimes is represented by two Spires, conically coiled, their apices directed away from or towards the centre of the shell (fig. 64).

Median Septum.-A partition that may be found running for some distance below the hinge-line towards the shell-border, rising from the inner surface of either valve, or both.

The shell-substance of the Brachiopoda (except the occasional phosphatic layers) is very characteristically built up of long curving calcite prisms, among which circular gaps, the perforations, commonly appear. The obliquity of these prisms to the surface of the shell, and their curving, allow their polygonal ends and their lateral faces to be visible at once in microscopic preparations.

A. ARTICULATA (Valves connected by a Hinge).

Shell Calcite.

Terebratula (fig. 59).-Shell oval, punctate; often folded slightly at the margin; surface smooth, with mere lines of growth parallel to the margin. Curved hinge-line. Beak pierced by a round foramen, the deltidium occurring below this and not sur rounding it. Brachial loop short.