944 PRINCIPLES OF STRATIGRAPHY 



time, when the number of specimens was considerably over a thou- 

 sand, they often acted as important reef-formers. Mesozoic and 

 Cenozoic Bryozoa (close to a thousand species) also contributed 

 largely to calcareous reefs. (See Chapter X.) The animal differs 

 from the coral polyp by the possession of a well-marked body cavity 

 and a definite alimentary system. 



The BracJiiopoda are simple animals encased in a shell with 

 dorsal ventral and sometimes accessory valves. In general, the 

 ventral valve is larger and some provision is afforded for the 

 emission through a foramen or otherwise of the fixing organ, or 

 pedicle. It is, hence, called pedicle valve. The other valve carries 

 supports {crura bracliidia), from which the soft internal respira- 

 tory organs, the brachia, or arms, are suspended ; hence the 

 natne hracliial Valve is applied. The accessory pieces are either 

 a third shell plate [pedicle plate, dcltidial plate) secreted by the 

 pedicle, or a double set of plates (deltidial plates) meeting in the 

 center below the foramen. These accessory plates are commonly 

 very small and situated below the beak of the pedicle valve. Open- 

 ing and closing of the valves is effected by muscular systems. Sur- 

 ficially the shells are either smooth or variously plicated, and 

 sometimes spines are developed. There are about 140 living and 

 over 6,000 fossil species. 



Phylum V — Mollusca. The molluscs are soft-bodied animals 

 generally enclosed in a calcareous shell. The headless molluscs, or 

 Felecypoda, have a shell of two, generally symmetrical valves placed 

 right and left and united dorsally by a hinge, which generally in- 

 cludes a series of interlocking hinge-teeth and sockets. The valves 

 are opened either by an external ligamental structure variously ar- 

 ranged or by an internal compressible resilium which often has 

 special supports or resilifers developed. The shell is closed by the 

 adductor muscles, of which there are typically an anterior and a 

 posterior one (dimyarian), or only one, situated subcentrally 

 (monoJu\arian). Externally the shell is smooth, showing only 

 growth lines, or it may be ornamented by radiating plications or 

 striations, or by marked concentric ribs parallel to the growth-lines. 

 A horny outer covering, or periostracum, is generally present. The 

 animal is provided with an anterior hatchet-shaped foot, and with 

 gills which hang in pairs on opposite sides of the abdomen, and 

 with a mantle, the attachment of which to the shell is marked 

 by the pallial line, and the outer portion of which secretes the shell. 

 The remainder of the mantle secretes the inner shell layer (nacre- 

 ous layer), which is often iridescent. A pair of siphons (excurrent 

 and incurrent) is frequently formed, their presence being generally 



