FOSSILS OF THE CAMBRIAN. 



37 



it is embryonic and representative of one of the earlier stages of growth 

 of the species. The second stage, of which an outline figure is given 

 on plate xxi, fig. 10, has changed somewhat, but the essential elements re- 

 main, and in fig. 1 1 there is a still greater expansion of the posterior margin 

 of the head and the genal angles are carried slightly forward. There are 

 probably several intermediate forms between figs 10 and 11 that have not 

 as yet been discovered, but for the present comparison they may be dis- 

 pensed with, as in fig, 1 i we have a form that may be compared in the 

 contour of the head with our figs. 1 and 3 ; the genal angles are forward of 

 the line of the posterior margin, and strong obtuse angles occur where the 

 facial sutures cut the latter at x x. In fig. 1 2 the normal adult form of the 

 head of this species is outlined. Reviewing the development thus hastily 

 sketched, it is seen that the form fig. 11 is intermediate between figs. 10 

 and 12, and representative of the group of forms found in O. Rowelli, be- 

 tween fig. 6 and the earlier forms preceding fig. 1 in size, fig. 6 being 

 considered as nearest the adult type in the contour of the head. Without 

 more data, it is, perhaps, conjectural to consider that the forms preceding 

 fig. 1 were like figs. 2 and 4, and that preceding these the earlier stages of 

 0. asaphoides were represented, the forms figs. 2, 4, 1,3,5 occupying the 

 position in development taken by 1 1 in the latter species. At present no 

 other view seems as reasonable, unless it is thought that fig. 2 represents 

 the extreme embryonic tendency of the species, a course of development 

 the reverse of that of Olenellus asapJioides, or any known species of the tri- 

 lobita, and one not at all probable in this instance. 



II. Eelations to other species and genera. The difficulty met with in 

 comparing the abnormal development of this species with that of other 

 species in which the thorax and pygidium are known is also felt in study- 

 ing its relations to various species and genera in which the structure of the 

 entire body is known, restricting as it does all comparisons to the head. 



The species associated with Olenellus Howelli are 0. Gilbert!, plate ix, 

 fig. 16, plate xxi, fig. 13, and 0. Iddingsi, plate ix, fig. 12. The smallest 

 head of the former, fig. 13, is about the size of fig. 2 of the same plate, but 

 does not present the strong embryonic features of the latter. In compari- 

 son with the adult head of the same species, plate ix, fig. 16, it is observed 



