i 5 o THE POPULAR SCIENCE MONTHLY. 



than nine centimetres in breadth ; the "young growth" were those 

 one or two years old. 



From these observations, which were made annually, Professor 

 Mobius discovers that there was on an average 421 medium oysters 

 to every 1,000 marketable ones that is, out of every 1,421 oysters 

 1,000 were full grown. The average of all observations differed very 

 little from the number given by each, and consequently shows that 

 there was but a slight fluctuation in the proportion in one hundred and 

 twenty -two years. The "medium" oysters are considered by Pro- 

 fessor Mobius to be those descendants of the " marketable " ones that 

 have survived their most precarious years of existence and escaped 

 their principal enemies, and are consequently likely to reach their full 

 growth. They thus represent the total number of oysters spawned 

 which have survived in the struggle for existence. From his experi- 

 ments Mobius decides that about one million eggs are spawned by 

 each oyster, and that about forty-four per cent, of the oysters on a 

 bed spawn each season. 



It is evident from the above that in an assemblage of one thousand 

 oysters four hundred and forty million eggs would be voided every 

 season, and of the resulting embryos four hundred and twenty-one 

 would survive ; or, 1,045,000 eggs would perish where one survived. 

 But the " medium " oysters also spawn, though they send forth a much 

 smaller number of eggs, and Mobius estimates that four hundred and 

 twenty-one in the community would produce about sixty million, or 

 the fourteen hundred and twenty-one would spawn together about five 

 hundred million eggs, and from these five hundred million only four 

 hundred and twenty-one oysters would be produced ; or, where one 

 oyster arrived at maturity, about 1,185,000 eggs or oysters perished ! 



As heretofore no examinations of the oyster-beds of this country 

 have been made, there is no data similar to the foregoing upon 

 which to base an estimate. As already pointed out, the mortality 

 among the American embryo oysters must be much greater than 

 among the European, and, in order that there might be some idea of 

 the decrease in number and increase in size of our own variety when 

 in the natural beds, there were deposited at various points in Tangier 

 and Pocomoke Sounds, Maryland and Virginia, during the summer of 

 1879, a number of earthenware tiles as " spat "-collectors. Many of 

 these were removed or destroyed, but from those left in place, by care- 

 fully counting at intervals the number of oysters attached, we have 

 been able to estimate the decrease in numbers during the early months 

 of existence. 



The inspections of the " spat " collectors showed that the oysters 

 continued attaching until about the 20th of August, and that the 

 largest number attached about the first of that month ; between the 

 23d of August and 10th of October the mortality was shown to be 

 fully fifty per cent. ; future examinations of the "spat "-collectors will 



