270 S. W. Ford— Embryonic Forms of Tribbites. 



the same. If, in the case of figure 2 we were to suppose that 

 portion of the ridge running from the lower extremity of the 

 eye-lobe which does not enter into the composition of the spine 

 of the general angle to be elastic, and then draw the lower end 

 of the eye-lobe downward and inward as close to the glabella 

 as possible, at the same time carrying the genal angle outward 

 a little way, we should get a short horizontal posterior margin 

 between the outer and inner spines. This is similar to what, I 

 believe, has, in effect, actually occurred jn the growth and de- 

 velopment of this form, the growth being both inward or toward 

 the line of suture and outward or toward the angle, giving us a 

 form such as we observe in figure 3. I have no doubt but that 

 future discoveries will bring to light forms illustrating this, and 

 also showing us more fully the various stages of unfolding 

 passed through in the development of this extraordinary species 

 of trilobite. 



II. Adult Forms. 

 Fig. 5 represents, twice enlarged, an unusually perfect speci- 

 men. It is of a young individual, but the characters show that 

 it was, without doubt, a fully developed form. The head is 

 somewhat crushed and fractured, but the main features are 

 beautifully shown. The thorax is incomplete, and the pygi- 

 dium is wanting. There are twelve pairs of ribs preserved. 

 These decrease in length in a regular manner, constituting an 

 exceptional feature in the structure of the genus.* In one of 

 the three specimens of this species figured in Dr. Emmons 

 works there are fourteen articulations in the thorax, but it is 

 impossible to say whether this is the total number possessed 

 by the individual. His figure also shows one of the posterior 

 spines of the head in place with its characteristic surface-mark- 

 ings.! In our specimen the whole surface of the head beyond 

 the eye-lobes and glabella is covered with irregularly alter- 

 nating finer and coarser lines radiating outward to the margin. 

 These lines are somewhat too coarsely represented in the figure, 

 but it shows well the peculiar character of the ornamentation. 

 In this specimen the slender ridges running from the eye-lobes 

 to the posterior margin along the sutural lines are very appar- 

 ent. The eye-lobes are not actus the glabella 

 behind as shown in the figure, which would also slightly change 

 the direction of the ridges extending from them. At a and o 

 the shield is fractured, but I do not consider either fracture as 



