266 THE CHICAGO ACADEMY OF SCIENCES. 



obliquely outward and backward, the second segments are much 

 smaller and are not grooved, their distal extremities make slight 

 projections upon the posterior margin of the pygidium, third 

 segments minute. 



The dimensions of a nearly complete individual are: total 

 length 24 mm., length of head 7.5 mm., length of thorax 14 mm., 

 length of pygidium 2.5 mm., width of head at posterior margin 

 15 mm., width of pygidium at anterior margin 7.5 mm. 



Remarks. This species is founded upon two nearly com- 

 plete individuals both of which were collected by Mr. L. H. Hyde 

 in whose honor the species is named. Some of the characters 

 mentioned in the description are best shown in one of the speci- 

 mens and some in the other one. The head of the species is 

 much like that of a small individual of C. niagarensis, but it 

 lacks the pitted surface of the cheeks and has the marginal 

 border continuing around the front of the glabella, the most 

 conspicuous difference between the two species is found in the 

 pygidium, C. niagarensis having all the pleural segments pro- 

 duced into spines, while C. hydei has only the anterior pair of 

 segments so produced. In the form of its pygidium the species 

 resembles C. pleurexanthemus of the Trenton limestone faunas, 

 but the cephalon is not tuberculate as in that species. The 

 specimen here figured is preserved in the collections of the Walker 

 Museum, the second specimen -being still retained in the 

 collection of Mr. Hyde. The greater portion of the outlines 

 restored in the illustration have be'en drawn in from Mr. Hyde's 

 specimen. 



Locality. Chicago Drainage Canal, near Lemont. 



Genus 17. SPHAEREXOCHUS Beyrich, 1845. 



Complete body elongate sub-ovate in outline. Cephalon 

 sub-globular, the cheeks much reduced ; glabella nearly spherical 

 with the two anterior pairs of lateral furrows nearly or quite 

 obsolete, the posterior pair deeply impressed, curving backward 

 to the occipital furrow and cutting off the sub-circular basal 

 lobes from the glabella; eyes small. Thorax with ten 

 segments, the axis prominent and bordered by strong axial 

 furrows; the pleural segments smooth, convex. Pygidium 

 smaller than the cephalon, the axis comparatively large, pleural 

 segments three with free points. 

 Sphaerexochus romingeri Hall, pi. xxiv, figs. 16-19. 



Description. Cephalon spheroidal, dorsal furrows strong 

 and deep, sloping downward from the posterior margin to points 



