PAPERS PRESENTED AT GENERAL SESSIONS 41 



phibia to which the frogs belong. This law was first dis- 

 covered by students of embryology, but the paleontolo- 

 gist has in some ways a better opportunity to test its 

 validity than the embryologist, especially as regards 

 stages in the life history somewhat later than the truly 

 embryonic. This is because, according to this law, the 

 mature shells of any age should be found to correspond 

 with immature growth stages of shells of their descend- 

 ants occurring in rocks of later age, and this has proven 

 true in a wonderful variety of fossil forms. An example 

 will suffice from the Cephalopoda, or animals which have 

 their shells separated into a number of chambers by par- 

 titions like the chambered nautilus. The earliest forms 

 with chambered shells were straight. Later, some of 

 these developed curved shells, and later still the loosely 

 coiled, and finally shells closely coiled in one plane, like 

 the Nautilus, were evolved. Xow the remarkable thing 

 is that when this closely coiled Nautilus shell is carefully 

 sawed lengthwise through the middle, it is seen that the 

 curvature in the oldest part or apical end of the shell is 

 not symmetrical. The shell begins to grow straight at 

 the tip, later becoming only slightly curved till the first 

 three septa are formed, then becomes loosely coiled, but 

 does not become closely coiled until the end of the first 

 volution. This remarkable manner of growth results in 

 leaving an empty space between the two halves of the 

 first volution, and repeats perfectly the order in which 

 the various degrees of curving and coiling of the Nau- 

 tilus type were developed successively in time. 



Now as if Nature was afraid this record was not suffi- 

 ciently clear, she has made the evidence of evolution 

 still more definite. In all of the closely coiled chambered 

 shells, like Nautilus, the septa or partitions were evenly 

 curved plates which joined the inner side of the shell 

 along straight regular lines called sutures. In Devonian 

 time there began to be developed in some of these shells 

 irregular wrinkling of the septa, causing the bending 

 backward and forward of the suture lines as in the Gon- 

 iatites. As time progressed the lobing of the suture lines 

 became more and more complex, as in the Ceratites, and 

 reached its culmination in the later Ammonites. During 



