556 GINSBUBG, LLOYD, STOCKMAN AND MCCALLITM [CHAP. 22 



boundaries. His suggestions and the views of Lowenstam (1950, 1954) were 

 the starting point for the following suggested grouping, which is based on the 

 relative resistance to breakdown of the whole skeleton. Only the more common 

 living members of each group are cited below, and no attempt is made to in- 

 clude all the sediment-producing organisms known. For data on the mineralogy 

 and trace-element composition of these skeletons, see Chave (1954, table 1) 

 and Revelle and Fairbridge (1957, table 8). 



A. Sheaths and Spicules 



This group includes all those species in which the mineral matter is so small 

 or so loosely held by organic tissue that, upon the death of the organism, it 

 disintegrates to fine particles. Most of these particles are of silt and finest sand 

 size, but others are finer; all are subject to further size reduction. 



Algae are the most common and abundant elements of this group (Fig. lA). 

 Many green algae (Codiaceae) have an external sheath of ^agonite crystals 

 (Figs. lA, B). The crystals are so loosely bound together that when the plant 

 dies and the organic matter decays, individual crystals or small aggregates of 

 crystals are released (Fig. IC). Some red algae (Fig. lA) release equigranular 

 particles of calcite when the organic matter decays. 



Many animals have mineral spicules within their external tissue; sponges, 

 alcyonarians (Fig. ID), tunicates, and holothurians are perhaps the most 

 common in Recent seas. When the animal dies and the organic tissue decays, 

 these small spicules are freed to be incorporated into the sediment. 



B. Segments 



This group, first suggested by Sorby (1879, p. 70), includes all those plants 

 and animals whose skeletons consist of segments held together in life by organic 

 tissue. When the organism dies and the organic linkage decays, the segments 

 separate to discrete particles most commonly of sand or gravel size. 



Green and red algae are the most common members of this group (Fig. 2A). 

 One green alga, Halimeda sp., is frequently the most abundant constituent in 

 shallow-water sediments. Dasycladacean algae (Fig. 2A) are also included in 

 this group. Although they are not particularly abundant in modern sediments, 

 they have been major contributors to carbonate sediments in the geologic past. 

 The articulated red algae (Fig. 2B) also produce segments, generally rod-shaped 

 and somewhat more fragile than the heavily calcified Halimeda. 



Echinoderms are the most familiar animal memliers of the segment group. 

 Crinoids, asteroids and ()])hiuroids all yield sand-sized fragments upon their 

 death (Fig. 2C). 



C. Branches 



Most plants and animals in this group have cylindrical branches such as those 

 of some species of red algae, corals and bryozoans (Figs. 2D, F, G). Others have 



