5 6 Field Museum of Natural History — Geology, Vol. IV. 



reports. B. orbicaudatus was originally described* from a pygidium only 

 and a complete specimen was afterwardsf figured by Billings. Re- 

 ferring to this description and figure, it is evident that B. beckeri is quite 

 distinct from B. orbicaudatus. The cephalon and pygidium are much 

 longer in proportion to their width, and the dorsal furrows are much 

 more distinct in B. orbicaudatus. The orbicular axis of the pygidium, 

 which is the distinguishing character of B. orbicaudatus, is missing in 

 B. beckeri. B. beckeri seems to be most closely related to B. billingsi 

 Raymond and Narraway from the Trenton limestone of Canada, but it 

 is considerably narrower in proportion to its length, the trilobation is 

 much less pronounced and no mention is made in the description of 

 B. billingsi of any transverse lines on the test. 



The specific name is given in honor of Mr. A. G. Becker, whose 

 collection contains the type specimen. 



Locality and horizon. — All specimens observed are from the Lower 

 Maquoketa beds of Clermont. 



. Genus THALEOPS Conrad 1843. 



"Ovate, profoundly trilobed, lateral lobes wider than the middle 

 lobe; buckler (cephalon) lunate, with very remote oculine tubercles, 

 not reticulated; abdomen (thorax) with 10-articulations; ribs without 

 grooves and not alternated in size; outer half of lateral lobes suddenly 

 depressed; post-abdomen (pygidium) without ribs or grooves and 

 profoundly trilobed." 



"This genus is remarkable for the great width of the buckler, and 

 the very prominent laterally projecting smooth oculine tubercles. It 

 differs from Bumastus in being profoundly lobed, and in having the side 

 lobes as in Asaphus much wider than the middle lobe. From Illanus 

 it may be distinguished by its ovate form, want of reticulated eyes, the 

 width of the lateral lobes, and the profound lobes of the tail." Proc. 

 Acad. Nat. Sci. Phil., 1843, Vol. I, p. 331. 



The above is Conrad's original description of the genus and to it 

 should be added that the free cheeks are produced laterally at the 

 genal angles into blunt spines. 



Thaleops ovata Conrad, Plate XIV, Figs. 6-8. 



T. ovata Conrad, 1843. Proc. Acad. Nat. Sci. Phil., Vol. I, p. 332. 

 Illcenus ovatus Whitfield, 1882. Geol. Wis., Vol. IV, p. 238, pi. 5, 

 figs. 1-2. 



*Can. Nat. Geol., Vol. iv, p. 379. 

 fCat. Sil. Foss. Anticosti, p. 27. 



