SECT. 3] SHAXLOW- WATER CARBONATE SEDIMENTS 559 



small diameter or thickness of their branches relative to their length, or because 

 of a delicate microstructure, they are quite fragile and are easily broken 

 mechanically or by organisms. It is, therefore, not surprising to find that even 

 where branching forms are the predominant sediment contributor, one rarely 

 finds entire unbroken skeletons in the sediment. Instead, fragments are the 

 rule. The size of the fragments depends primarily on the size and the strength 

 of the branches and secondarily on the nature and intensity of organic and 

 mechanical breakdown to which they are subject (Figs. 2D, E). 



D. Chambers 



All the skeletons that are hollow, or partly hollow, are included in this group 

 — gastropods, Foraminifera, pelecypod valves, echinoderms, worms, brachio- 

 pods and some crustaceans. These chambers persist after the death of the 

 animal, but there are wide variations in the resistance of different types of 

 chambers to further breakdown. The strength of the chamber is determined by 

 its absolute size, wall thickness, shape and microstructure. The persistence of 

 thin-walled foraminiferal shells illustrates how size and specific gravity can 

 prevent mechanical breakdown of the foraminiferal chamber. The effect of 

 shape is clear when we consider the mechanical breakdown of a flat pelecypod 

 valve and a gastropod shell of similar wall thickness. The pelecypod shell can be 

 broken easily, but the curving gastropod shell will not break so readily. The 

 echinoid test (Fig, 3C) is included in this class, but the spines which separate 

 on death are classed as segments, 



E. Crusts 



This group includes all those plants and animals that encrust sm"faces — 

 species of algae, corals, annelid worms, Foraminifera, bryozoans, hydrocorallines 

 and others. Most of these forms are well attached to their substratum and their 

 breakdown is dependent mainly upon the breakdown of the substratum. 



Where encrusting forms are on an organic substratum, such as grass (Figs. 

 3D, E), algae or alcyonarians, they will break down to finer particles when the 

 host dies and decays. More often, however, encrusting forms occur on larger 

 masses of hard mineral matter, usually corals, rocks or other resistant surfaces 

 (Fig, 3F). These encrusting forms are quite persistent and are reduced in size 

 only when the host is broken or disrupted. 



F. Massive 



Colonial corals, particularly those that form large hemispherical heads, are 

 the predominant members of this group (Fig. 4A), They are quite resistant to 

 breakdown by virtue of their size and the cellular nature of their microstructure. 

 Some coraUine algae that start as encrustations develop a massive form (Emery, 

 Tracey and Ladd, 1954, p. 27) and, like the corals, are quite resistant to break- 

 down {ibid., pis. 14-1, 16-1). 



