365 
R. T. Jackson has accomplished the same result for the Pelecypoda 
by following the same mode of analysis, and shown that Nucula was 
the common form to which all bivalve shells can be traced. Among 
corals, as shown by Beecher, there are satisfactory indications that 
there is a common ancestral form of at least a large proportion of 
that class, and the labors of Barrande, Mathews, Walcott and 
Beecher are leading to similar conclusions for the Trilobites. The 
theory of monogenesis, or origin of similar forms from one form, 
is in other words now rapidly passing from the condition of a rea- 
sonable inference from the facts of development and evolution, in 
which it has stood since the time of Von Baer, to that of a demon- 
strated law of general application. 
The individual coiled shell of every nautiloid may be said to 
pass through the stages of the protoconch and point of the apex, 
when it is nearly straight ;* then it becomes slightly curved or cyr- 
toceran, and then through a more completely curved or gyroceran 
stage, in which the first volution of the spiral is completed. After 
this it continues the spiral, commonly revolving in the same plane 
and becomes truly nautilian, the whorls on the outside touching the 
exterior of the inner ones, and spreading so rapidly by growth as 
to begin to envelop them, and in extreme cases, as in JVautilus 
pompilius, completely covers them up. 
The natural inference from these facts would be, that there was a 
similar succession of forms in past times—the straight in the most 
remote, the arcuate and the gyroceran in succeeding periods, and 
the nautilian only in comparatively modern times. This would be 
a perfectly clear and legitimate mental conception. ‘The structural 
relations of the adult shells appeared also to demand the same solu- 
tion, as shown by the researches of Quenstedt, Bronn and Barrande, 
and later of Gaudry. Barrande’s researches, however, demonstrated 
that this idea could not be maintained, and that there were no such 
serial relations in time, but that the whole series of forms from the 
straight to the nautilian were present in the earliest period, and 
occurred side by side in each Paleozoic formation. 
This great author’s conclusions have had a curious effect upon 
*It is to be noted in this connection that the earliest nepionic substages do not have 
equal circular bands of growth, even in true Orthoceras, and are never quite symmetri- 
cal on the dorsum and venter. In other words, the descriptive term, straight, is only 
applicable in a general way. The youngest stages of the conch having differentiated 
venter and dorsum and a compressed elliptical outline which is similar to that of the 
radical ancestral form Diphragmoceras. See Figs. 10-12, p. 361. 
