42 BULLETIN OF THE NATURAL HISTORY SOCIETY. 



The distinction of the headshields of the two species come at a 

 later period of growth. Both, in the younger stages (though not in 

 the youngest) have an apical spine on the margin of the headshield ; 

 this spine continues on the headshield of the Oleniod form, whose 

 growth is arrested, so that it never reaches the size of the Anomo- 

 caroid form. In this latter the apical spine is gradually absorbed, so 

 that in the final moult there is no trace of a spine in the front of the 

 shield. 



We see from this example how not only two species, but two 

 genera, may be developed from the same rootstock. That is, the 

 differentiation may produce two separate genera direct from the 

 phylum, without the indirect method of producing separate species 

 before the generic characters appear. 



To make this instance of development more easily understood the 

 diagram on the preceding page has been prepared, showing the branching 

 of the trilobite Acantholenus spiniger from the stock which eventuated 

 in Anomocare stenotoides. From this diagram it will be seen that the 

 divergence occurred in very small heads, for even the caudal shield, 

 1 mm. long, of the former species has all the rings of the adult, hence 

 the differentiation must have occurred in a still earlier moult. We 

 assume that this may have been the |mm. size. But if so the com- 

 panion species A. stenotoides must have had an equally small pygidium 

 as a starting point ; so the differentiation must have occurred as early 

 as the 1 mm. moult (i. e., when the head and caudal shield together 

 were of this length). 



But, as the diagram shows, the head-shields of the two species 

 cannot be distinguished until they reach a size of 2|mm. Had we 

 perfect tests, however, showing the head and caudal shields in connec- 

 tion, we should be able to distinguish the two trilobites by the pygidia 

 when this part was no more than ljmm. long, but failing this the 

 separation of the head cannot be effected before the 2Jmm. moult. 

 In the very smallest size the spine at the front of the shield (apical 

 spine) cannot be found, but immediately thereafter becomes a distin- 

 guishing feature, and continues so through all the earlier muults. But 

 by degrees it becomes absorbed in Anomocare stenotoi<!>'*, as the diagram 

 shows, but in the other species it continues as a distinct ornament until 

 the full size is attained, which, however, is much below that of the 

 species last named. 



As no other type of trilobites that can be confounded with them 

 occurs with these, it is clear that we have here a case of development 



