Geology and Natural History. 421 



alternation be increased, the pain will be diminished. The authors 

 state in conclusion that mere statements in regard to voltage 

 unaccompanied by statements in regard to current are highly- 

 misleading. — Electrical Meview, Sept. 12, 1890. j. t. 



II. Geology and Natukal Histoky. 



1. Phytogeny of the Pelecypoda, the Aviculidce and their 

 allies / by Robert Tracy Jackson, S.D., Mem. Boston Soc. 

 Nat. Hist., vol. iv, no. viii, pp. 277-400, pi. xxiii-xxix, 53 figures 

 in the text, July, 1890. — Each time a familiar subject is studied 

 from a new standpoint, many novel and interesting results may 

 be expected. In the present instance, the author has employed 

 modern and approved scientific methods, and the results, while 

 both novel and interesting, are of the highest importance to a 

 proper understanding of the pelecypods. The leading method 

 which is here so fully applied is that of a study of the stages of 

 growth. The embryology and anatomy are constantly kept in 

 view and also the chronological history of each group in past 

 geologic time. If all these aspects of growth can be brought 

 into harmony, we have the strongest evidence of the accuracy of 

 our observations, and most reliable taxonomic data. 



Professor Hyatt in his studies of the stages of growth and 

 decline among the cephalopods has constructed a model, and 

 indicated methods which may be profitably applied to all branches 

 of natural history. These principles have been followed by the 

 author, although some particulars have been slightly modified in 

 order to adapt them directly to the pelecypods. A new term is 

 proposed for a stage of growth between the typembryo of Hyatt, 

 which as re-defined is characterized in mollusks by a shell gland 

 with an initial plate-like shell, and the period showing a completed 

 protoconch. This intermediate period the author terms the 

 phylenibryonic, or that in which the shell and anatomy are each 

 sufficiently differentiated to determine the class to which the 

 organism belongs. 



The important discovery of the characters and relations of the 

 larval or embryonic shell named the prodissoconch was briefly 

 described in a previous paper by the author, but is here fully 

 treated in its relations and significance in the class. Its existence 

 is demonstrated in about thirty genera belonging to widely 

 differing families of pelecypods, recent and fossil, and is believed 

 to indicate a primitive ancestral condition common to the whole 

 class. The consideration of the oyster and allied forms com- 

 prises one of the leading features. The development of the 

 animal and shell is described and illustrated in over thirty pages 

 and three plates. It is shown that the ostreaform shell is due to 

 the cemented condition of fixation, and on this account is closely 

 simulated in other attached shells, in genera and families which 

 are not closely genetically related. In Pecten, the study of the 

 habits and anatomy at different growth-stages shows the intimate 



