36 PALEONTOLOGY OF THE EUEEKA DISTRICT. 



in some other species, a few of the questions arising from the study of the 

 stages of growth observed will be spoken of as they are shown in the first 

 nine figures on plate xxi. 



It is impossible to imagine that the course of development was by size 

 through figs. 1, 2, 3, 4, and 5; but if we take out the two forms figs. 2 and 

 4, those remaining, figs. 1, 3, 5, and 6, constitute a very good advancing 

 series and the larger specimens, figs. 7, 8, 9, might be considered as illus- 

 trating the principle that the size is not proportional to the stage of develop- 

 ment. The two forms taken out, figs. 2 and 4, however, prevent the first, if 

 we consider them as belonging to the same species, as appears to be the 

 case. They are associated with such forms as 3, 5, &c., and the two species 

 Olenellus Gilbert^ plate ix, fig 16, plate xxi, fig. 13, and O.Iddingsi, plate ix, 

 fig. 12, in the same hard specimens of arenaceous shale, and, although fig. 

 2 is an extreme form, fig. 4 is a gradation in form to such forms as figs. 1 

 and 3. It is observed that figs. 7, 8, and 9 precede 6 in the stage of devel- 

 opment of the contour of the head, at the same time showing a more accel- 

 erated development in the form of the eye. The eye of fig. 5 is more ad- 

 vanced than that of fig. 6, while the posterior outline of the head is more 

 embryonic in its character. With these examples it is readily conceived 

 that figs. 2 and 4 are enlarged examples of the forms that preceded fig. 1 in 

 development, and the eye of fig. 2 supports this view, as it is, in its strongly 

 developed ocular ridge, which is more marked than in fig. 1, and more an- 

 terior in relation to the frontal lobe of theglabella, essentially more embryonic; 

 and the position of the genal angles and spines is either a- decided embryonic 

 feature, or a sport that, with fig. 4, to connect it with figs. 1 or 3, would not 

 be considered as probable. 



Was, then, the probable course of development of this species, before 

 it reached the character of fig. 1, through earlier stages to a form like fig. 2, 

 thence to figs. 4, 1, 2, 5, and so on to the adult? 



In the development of Olenellus asaphoides, as given by Mr. Ford, 3 the 

 youngest stage is about 2 mm in diameter and nearly circular in outline, the 

 genal angles and spines are situated close in by the side of the facial suture, 

 as it is subsequently developed, and there appears to be no doubt but that 



8 Amer. Journ. Sci., 3rd ser., vol. xiii, p. 265, figs. 1-4, 1877. Ibid., vol. xxii, p. 250, 1881. 



