574 



DIKELOCEPH ALUS OF MINNESOTA. 



The form of the eye has not been satisfactorily ascertained, but it is probably similar to that of Ogygia. 



Number of thoracic segments only indistinctly indicated by detached fragments, which, together with 

 analogy of form, render it probable that they were eight in number. With the exception of one or two 

 specimens, occurring in the Lower Magnesian Limestone, F. 2, all the individuals and species of this 

 genus have been found in the Lowest Sandstones of Wisconsin, Iowa, and Minnesota, F. 1. 



DIKELOCEPHALUS MINNESOTENSIS. (N. S.) 



(Tab. I., figs. 1, 2, 10; Tab. I. a, figs. 3, 6.) 



Specific character. — The principal distinctions between this and the other species of the genus are its 

 larger size, less convexity of the glabella, the greater extent and slightly concave, shovel-shaped area 

 expanded in front of the glabella ; with the greater size of the caudal flap and the shortness of the caudal 

 spines, set wide apart, and projecting vertically backwards ;* five or sometimes six segments in the axal 

 lobe of the pygidium, since the terminal and largest lobe is usually, though obscurely, divided by a faint 

 furrow. 



Dimensions. — Cephalic shield 1^ long; caudal shield 2 T 3 r > long, 4 inches wide. 



This species was first found, and is most common, in a dark gray, argillo-calcareous bed intercalated in 

 member d of F. 1, ninety or one hundred feet below the base of the Lower Magnesian Limestone, near the 

 margin of Lake St. Croix, above Stillwater ; towards the base of La Grange Mountain, and at the Great 

 Slide, below Lake Pepin, on the Mississippi, which is the fifth Trilobite-bed of the series in F. 1. See table 

 on page 52. 



DIKELOCEPHALUS PEPINENSIS. (N. S.) 

 (Tab. I., fig. 9, a and 6, fig. 13(?), and Tab. I. a, fig. 17 (?).) 



Specific character. — The glabella of this species is divided into three segments, without any partial 

 furrow on the anterior lobe, which is relatively larger than in the preceding species ; the other two are 

 contracted into two narrow, basal segments. The facial sutures run almost parallel to the glabella, 

 making a slight, sigmoid flexure, and leave but a very scant area between them and the glabella, which, 

 in front, is almost in contact with the anterior, thickened border. 



The caudal shield found associated (fig. 9, b), and which appears to belong to the same species, is small, 

 highly arched ; axal lobe wider in proportion than in the preceding species, with five articulations ; border 

 narrow. 



Dimensions. — Cephalic shield -fo inches deep ; caudal shield about the same depth, and j 9 n wide. 



This species is found on the banks of the Mississippi River, in the buff, dolomitic layers near the top of 

 member d of F. 1, near the base of La Grange Mountain, at the head of Lake Pepin. 



Fig. 7, Tab. I. A, found associated in the same bed, is, most probably, the cheek- or wing-plate of the cephalic 

 shield of this species, which is provided with lateral spines of great length. Part of the cephalic shield 

 of another Trilobite has been found very analogous to this, and perhaps a variety of the same species, as 

 it appears to differ only in having a narrow area in front of the glabella, between it and the thickened anterior 

 border of about T 2 D ths of an inch wide. 



DIKELOCEPHALUS MINISCAENSIS. (N. S.) 



(Tab. I., figs. 3 and 12, and Tab. I. a, figs. 4 and 5.) 



This species is, at present, only known by its pygidium, and a portion of the cephalic shield represented 

 by fig. 3, a, b, Tab. I., and fig. 5, Tab. I. A. It differs from the D. Minnesotensis in being of smaller dimen- 

 sions ; in the glabella being more highly arched and elongated ; in the cephalic shield being more rounded 



* See fig. 2, Tab. I., where they are entire; in fig. 1, they are broken off. 



