Subsurface Laboratory Methods 95 



life cycle. Each series of molt valves generally differs somewhat from 

 the preceding one. This factor must be carefully considered by micro- 

 paleontologists in speciation work, as classification of fossil ostracodes 

 depends primarily on shell characteristics. Living forms are commonly 

 identified on the basis of the animal appendages. 



The most important elements of fossil ostracodes in descriptive work 

 are (1) general shape and outline (side, dorsal, ventral views), (2) rela- 

 tive size and overlap of the valves, (3) surface ornamentation, (4) hinge 

 characteristics, (5) muscle-scar pattern, and (6) characteristics of the 

 interior marginal zones. 



Before a species can be properly described, it is also necessary that 

 the male and female individuals be distinguished. This distinction is not 

 always possible because of the rarity of valves and slight differences in 

 shell structure of the opposite sexes. 



Fossil carapaces may be obtained from the enclosing sediment by 

 extraction methods mentioned under "Foraminifera," although the screen 

 series generally involves only the 60- and 100-mesh units. 



In determining the vertical and horizontal distribution of ostracodes 

 in stratigraphic sequences, the same procedure is followed as in foramini- 

 feral studies. Charts are prepared showing species occurrences and abund- 

 ences plotted against stratigraphic position. These data are then evaluated 

 in terms of zonal intervals. 



Calcareous Algae ^ 



Algae are seaweeds. Some have the ability to secrete lime around 

 or within their tissue, and, hence, may be preserved as fossils. These are 

 known as the "calcareous algae." During many times in the geologic past 

 in many places they have grown so luxuriantly as to build or largely 

 build extensive deposits of limestone.^ 



Frequently fragments or even entire specimens occur in well cuttings. 

 They can be separated and concentrated in the same manner as most other 

 microfossils. Most of them are about the size and shape of fusulinids 

 (fig. 41). 



There are a number of groups of calcareous algae, but only three, 

 the coralline algae (Corallinaceae) , the siphonous algae (Dasycladaceae), 

 and the Charophyta (Chara) are of economic value at present as micro- 

 fossils. 



Corallinaceae 



The Corallinaceae (fig. 41, no. 5; fig. 42) are a family of the red 

 algae. They develop a very large number of growth forms, the most com- 

 mon being (1) forms with thin crusts, (2) forms with crusts from which 

 rise mammillary protuberances or small stubby branches and (3) branch- 

 ing forms. The branches may be quite extended and may develop as 



° The section "Calcareous Algae" was prepared by J. H. Johnson. 



'Johnson, J. H., Limestones Formed by Plants: Mines Mag., vol. 33, pp. 526-533, 1943. 



