Section 1. Thecosomata.--Shell or Cartilaginoid Test Always Present,

fins united by an intermediate lobe, ctenidia as a rule absent, replaced by secondary branchiae, no very distinct head or eyes, one pair of tentacles; cerebral ganglia on the sides of and under the oesophagus; radula with three rather large teeth in a row, generally unicuspid, jaw in two pieces, stomach with horny plates, anus generally on the left side.

The Thecosomata feed on Protozoa and the lower Algae; they have no proboscis, and the intestine is flexured. The fins are always closely connected with the head, or what answers to it. About 42 species are known, belonging to 8 genera.

FAM. 1. Limacinidae.--Fins very large, branchial chamber dorsal, anus on right side; shell spiral, sinistral (ultra-dextral, see p. 249), operculate. Genera: Limacina, shell helicoid, deeply umbilicated (L. helicina swarms in Arctic seas and furnishes food for many Cetacea); Peraclis, spire turreted, aperture large, elongated, produced anteriorly, no umbilicus; operculum sinistral, in spite of the shell being ultra-dextral.

FAM. 2. Cavoliniidae.--Fins large, branchial chamber ventral, shell a non-spiral cone, angular or round, very thin, embryonic portion distinct, or formed of two separate plates.

In Cavolinia (= Hyalaea, Fig. 5 B, p. 10) the shell consists of two plates, the ventral being convex, with one to three sharp spines at the posterior end, the dorsal flatter, without spines. The aperture is broad, contracted dorso-ventrally. Two long pointed prolongations of the mantle project from the lateral slits of the shell, and probably serve to balance the bulky body when swimming. Fins trilobed at the margin. Cleodora has only rudimentary lateral prolongations, fins bilobed, shell triangular, angles greatly produced, aperture very wide, dorsal side keeled. In Cuvierina the shell is straight, sub-cylindrical, with a median partition, slightly expanding towards the apex, which is truncated in the adult. The principal sub-genera of Clio are Creseis, which has an elongated sub-cylindrical shell, sometimes slightly curved, smooth or grooved; and Clio proper, in which the shell is long, angular, with a dorsal rib, apex (= embryonic shell) rounded, constricted. Styliola and Hyalocylix also belong to this group.

FAM. 3. Cymbuliidae.--Test (which is not homologous with the shell of other Thecosomata) slipper-shaped, cartilaginoid, simply a thickening of the mantle; embryo with a calcareous, spiral, operculate shell. Genera: Cymbulia, Cymbuliopsis, Gleba.

Three other families, Hyalithidae, Pterothecidae, and Conulariidae, from Palaeozoic strata, are generally added to the Thecosomata. All are fossil only, and it is doubtful whether they are really Molluscan. Pelseneer holds that no true fossil Pteropoda occur until the lower Tertiaries.