CLASS II • BRACHIOPODA 367 



protegulum, and showing no separate or distinct stages of growtli, is represented by 

 tlie genus Paterina (Fig. 543). 



So far as observed, the protegulum, or Paterina stage, in tlie Atremata and 

 Telotremata is followed by tlie Obolella stage of nearly circular outline. After this 

 stage, specific cliaracters appear, and in tlie open delthyrium of tlie Telotremata there 

 are usually developed the first rudiments of tbe deltidial plates. In the Protremata, 

 the Paterina stage is not followed by the Obolella stage, but the wide delthyrium of the 

 protegulum is at once affected and modified, and develops into the deltidium. In the 

 Acrotretacea, belonging to the Neotremata, there is often developed a homoeodeltidium, 

 resulting from secretion by the mantle, and therefore of different origin from the true 

 deltidium occurring in the Protremata, which is deposited by the pedicle. In the 

 Discinacea, belonging to the Neotremata, the pedicle opening is an open notch in the 

 posterior margin of the ventral valve. In derived forms this is progressively closed 

 posteriorly ; geologically in the phylum, and ontogenetically in the latest derived 

 genera and species. 



Habitat and Distribution. — Brachiopods are usually gregarious in 

 habit, often growing in Clusters attached to one another. This is not only 

 true of Recent species, but of Paleozoic forms as well. Brachiopods are 

 found in all latitudes and at all depths, but are largely shallow-water animals, 

 for of the living species 71 per cent occur between the strand-line and 100 

 fathoms. Liothyrino, wyvillii was dredged from the enormous depth of 2945 

 fathoms. Terebratulina caput-serpentis ranges from a few fathoms to a depth 

 of 1170 fathoms. 



Brachiopods are most abundant in warmer seas, the Japanese province having 

 twenty-nine species. As a rule, those occurring in cold waters are not found 

 in warm waters. Lingulids and Discinids are almost restricted to the strand- 

 lines in warmer waters less than 60 feet deep. Of the thirty-three living 

 genera, at least 60 per cent have fossil representatives. Lingula and Crania 

 have lived since the Ordovician ; since the Jurassic, six genera have continued, 

 since the Miocene one, since the Pliocene seven, and since the Pleistocene three. 

 Of the 158 living species only 16 per cent occur fossil, and but five are as old 

 as the Eocene and Miocene. Three genera are confined to the deep sea, and 

 all of the abyssal forms are usually thin-shelled, brittle and translucent. 



Migration of Brachiopods is possible only during the early larval stages, 

 and then to a very limited extent among the articulate forms. Morse observed 

 that Terebratulina became attached in a few days, but Müller kept Discinisca 

 in confinement nearly a month before any became sessile. 



Colour. — The shells of most living species are of light or neutral tints, 

 white or horn-colour. A deep orange-red in radiating bands or in solid tints 

 colours some species (Terebratulina, Kraussina, etc.) ; light yellows, deep and 

 light shades of green (Lingula), black in bands (Crania), or masses (Rhynchonella) 

 embellish these shells. Even among the fossil species traces of faded colour- 

 marks are occasionally observed ; Deslongchamps has described them among 

 Jurassic species, Davidson among the Carboniferous, and Kayser has found a 

 colour-marked Rhynchonella in the Devonian. The large highly ornamented 

 species of Paleozoic times, with their external sculpture heightened by a 

 brilliant colouring, must have been objects of exquisite beauty (Hall and 

 Clarke). 



Classification. — The Brachiopoda, since 1858, have been divided by 

 nearly all systematists into two Orders, based on the presence or absence of 



