J. IMBRIE 



141 



population. For example, in a large collection of the Silurian 

 ostracod Beyrichia jonesi analyzed by Spjeldnaes (1951) there is 

 a progressive depletion in frequency at least from the fourth to 

 the eleventh instars (Fig. 9). This pattern has been taken as evi- 



> 

 o 



o 



UJ 



o 



30 



20 



10- 



Micraster 



N = 348 



27 33 39 45 51 57 63 



Strophodonta 



Crurithyris 

 N * 511 



J] 



B 



V 



3j0 4.0 5.0 6.0 7.0 ao 



Strophudonta 



295 375 455 535 



6 8 10 12 14 



SIZE IN MM 



Fig. 10. Size-frequency distributions of four typical samples of fossil 

 invertebrates. N, number of measured specimens; A, measurements of great- 

 est width in a sample of the echinoid Micr aster coranguimim from the Cre- 

 taceous of Northfleet, England. Data from Kermack (1954). B, measure- 

 ments of maximum width in a sample of the brachiopod Crurithyris plano- 

 convexa from the Dry Shale of Kansas. Data from Tasch (1953). C, meas- 

 urements of maximum width of the brachiopod Strophodonta sp. from a 

 single locality in the Gravel Point formation, Michigan. Original data. D, 

 measurements of maximum length in a sample of Strophodonta extenuata 

 ferronensis from a locality in the upper Ferron Point formation, Michigan. 

 Original data. 



dence that the collection approximates a random sample of the 

 original population, and on this assumption Kurten ( 1953 ) has 

 calculated life tables giving mortality rates at each growth stage 

 as well as other standard parameters of population dynamics. 



