380 



of denudation, leaving coves, where it can be measured, and 

 where it is found to be only thirty or forty feet thick, agreeing 

 with actual measurement in other places ; whereas, by inferential 

 measurements, its thickness has been made several miles. 



Dr. W. I. Burnett read a communication on the Develop- 

 ment of Viviparous Aphides, as follows : 



The peculiarities attendant upon the reproduction of plant-lice, 

 have long been familiar to Naturalists, and, from the earliest 

 times, have excited the attention and curiosity of Zoologists and 

 Physiologists. These peculiarities consist briefly in the fact, that 

 there is here a Lucina sine concuhitu ; whole colonies of these 

 insects, consisting exclusively of individuals capable of bringing 

 forth their kind, and this, without there being among such colo- 

 nies any male individuals. The authenticity of these wonderful 

 conditions of reproduction, has been satisfactorily established in 

 science, by the queer and successful experiments made by many 

 of those quaint Entomologists of the last century. A brood of 

 plant-lice appears early in the spring, as soon as the sap begins 

 to flow freely ; this brood consists solely of individuals, which are 

 reproductive, and each of these last has developed within it a 

 certain number of new individuals, which, like their parents, are 

 all of the reproductive kind, there being no males among them. 

 These individuals follow the same course as their progenitors, so 

 that each successive brood exceeds the previous one, in a certain 

 geometrical ratio. This constant succession of broods continues 

 through the whole summer, there being sometimes seven, or nine, 

 or eleven, according to the season. In the autumn, the last 

 brood which appears, consists of regular male and female indi- 

 viduals. The females soon lay eggs, after which, they die ; the 

 eggs hatch, and the individuals thereby produced enter the earth, 

 where they pass the winter, reappearing in the ensuing spring, 

 constituting the first brood of that season, and reproducing their 

 kind, &c., as already described. 



My researches were made with a view to ascertain the inti- 

 mate nature of these remarkable reproductive phenomena ; they 

 were commenced this last spring, and have recently been com^ 



