38 PALEONTOLOGY OF THE EUREKA DISTRICT. 



that the eyes in the smaller individual are a little distant from the glabella and 

 united to it by a very short ocular ridge ; otherwise no embryonic features 

 are indicated. The example of the head of 0. Iddingsi is still smaller, but 

 it does not present any recognized embryonic features. 



Olenellus Gilberti and 0. Howelli also occur at the same relative geologic 

 horizon, at Pioche, Nevada, 130 miles (209.2 kilometers) distant. The for- 

 mer species in that locality is noticeable in having, in the smaller specimens 

 observed, an obtuse angle in the posterior margin of the head midway 

 between the genal angle and the glabella (see outline, fig. 14, plate xxi). In 

 the larger specimens the angle is near the outer portion of the margin or a 

 little within the postero-lateral angle. The facial suture back of the eye 

 is quite unlike that of 0. Howelli, as shown for the latter in figs. 5 and 6, 

 and cuts the posterior margin between the angle, x x, of the margin and 

 the glabella, as seen in figs. 14 and 17. In comparing with 0. Thompsoni and 

 0. Vermontana Hall, from the Georgia slates of Vermont, it is only to be 

 observed that in the former species the general features of the adult, as far 

 as known, appear to be the same, and that in the latter the facial suture has 

 the same course back of the eye as in 0. Gilberti, and Prof. R. P. Whitfield 

 has shown me a specimen in the collection of the American Museum of 

 Natural History that is marked by a very short spine on the obtuse, rounded 

 angle of the posterior margin, a short distance within the postero-lateral 

 angle, a character not observed in any other species of the genus. 



Mention has been made of Olenellus asaphoides and certain resemblances 

 in the contour of its head at the stage of development represented by fig. 11, 

 and that of the head of 0. Howelli as seen in figs. 1, 3, 5, etc. The curious 

 interocular spines of the former have not been seen in 0. Howelli. Mr. Ford 

 has called attention to the Paradoxides-like run of the posterior margin of 

 the head, g x, xg, fig. 11, and states that it disappears altogether during the 

 embryonic life of that species. We have shown that it is extravagantly 

 developed in 0. Howelli, even to the extent of changing the entire contour 

 of the head, fig. 2, and that it persists in the adult stage of many individuals 

 of this species, and is also present in 0. Gilberti and 0. Vermontana. 



In all the observed specimens of 0. Howelli showing the facial suture 

 back of the eye the posterior margin is cut by it at the angle within the 



