﻿£65 



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 cf 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 Nautilus 

 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. 



