Geol.— Vol.. I.] SMITH— LYTOCERAS AND PHYLLOCERAS. 131 



order, for each stage of growth represents some extinct 

 ancestral genus. These genera appeared in the exact order 

 of their minute imitations in the larval history of their 

 descendants, and by a comparative study of larval stages 

 with adult forms the naturalist finds the key to relation- 

 ships, and is enabled to arrange genera in genetic series."^ 

 The method used in this investigation is that given in the 

 paper cited above; it was first discovered by Hyatt, and 

 afterwards used by Branco with such valuable results. By 

 this method a series is not pieced out by selecting a number 

 of separate individuals supposed to represent successive 

 stages of the same species, but a number of well preserved 

 individuals is selected, large enough to make sure of the 

 identity of the species, and the specimens are reduced by 

 pulling off the outer coils to any stage desired. The old 

 way was to mount the specimens in Canada balsam on a 

 glass slide, but this is unsatisfactory, for it allows a view in 

 only one direction. The writer has found it much better to 

 keep the small specimens separate in labelled tubes, and to 

 use three different mounts in microscopic study; dry on 

 white cardboard, to study the shape and surface markings; 

 in a drop of water spread out so as not to distort the object, 

 to show the septa; under water in a watch-glass over a 

 strong condensing lense, to study the siphon, inner septa, 

 and other organs when the cast or shell is translucent. 

 The viscosity of water will hold the minute object in any 

 desired position, so a fixed mount is unnecessary. 



Nomenclature of Stages of Growth. 



In order to correlate stages of growth of the individual 

 with generic stages seen in the development of the race, it 

 is necessary to have an exact scientific nomenclature. The 

 most satisfactory is that given by Professor Hyatt in " Phy- 

 logeny of An Acquired Characteristic." ^ 



1 Journal Geol. Vol. V, 1897, No. 5, J. P. Smith, "Comparative Study of Palaontogeny 

 and Phylogeny," p. 517. 

 - Proc. Anier. Phil. See, Vol. XXXII, No. 143, pp. 391 and 397. 



