FOSSILS OF THE CAMBItfAK 



earliest embryonic condition with which we are acquainted to the adult 

 form is marked by the disappearance of the embryonic characters one by 

 one as the individual increases in size and assumes more and more the feat- 

 ures of the fully developed animal, all of which usually takes place, except 

 in size and surface ornamentations, when it is quite small. The retaining 

 of an embryonic feature after the individual has passed in size, or any other 

 character, the stage at which it usually disappears in the regular course of 

 the development of the species, is an exceedingly rare occurrence, and is 

 unknown, to our own knowledge, except in the increase in size of the body 

 in certain species, so that in their course of development certain individuals 

 are in fact larger than those of the same species having a greater number of 

 segments in the thorax and consequently more advanced in development. 

 M. Barrande has shown this to be the case in the development of Arethusina 

 Konincki, Proetus decorm, and P. venustus* and we have observed it in 

 Triarthrus Beckij where the relative size, proportional to the development, 

 is very marked, e. g., an individual with 13 segments in the thorax is 24 mm 

 in length, and one with 16 segments but 16.5 mm long, while the fully-grown 

 example of 16 segments reaches a length of 53 mm , and some with 13 seg- 

 ments are but 7 mm in length. 2 



This peculiarity of growth is shown in the species mentioned only by 

 the thorax, for if we take the head apart from it there is little or nothing to 

 prove that its size is not proportional to the stage of development; but in 

 Olenellus Howelli the head proves this to be otherwise, and there may be added 

 to the statement, that in certain species the size is not proportional to the num- 

 ber of liberated segments in the thorax, that in other species the size of the 

 head is not always proportional to its stage of development. It is greatly 

 to be regretted that the thorax and pygidium are not preserved, so that a 

 direct comparison of those parts could be made with abnormal conditions 

 of development in other genera and species. 



Having shown that the peculiar development of this species, exhibited 

 in the acceleration and retardation of growth in individuals as compared 

 with each other, is in a measure comparable with the peculiarities of growth 



'Systeme Silurien de la Bohfime, vol. i, p. 268, 1852. 



"Trans. Albany Inst., vol. x. Fossils of the Utica Slate, p. 29, 1879. 



