50 Field Museum of Natural History — Geology, Vol. IV. 



position of the facial sutures. In /. iowensis they follow the anterior 

 margin of the cephalon and unite in a curve or indistinct angle. In 

 /. maximus the sutures run subparallel to the anterior margin and meet 

 in a distinct angle so that the free cheeks are produced in front of the 

 glabella, while in /. iowensis the cheeks terminate in front of the eyes. 

 Owen describes the genal spines as extending the entire length of the 

 thorax, but his original figures show the spines much shorter. In the 

 specimens before the writer the length of the spines seems to be a 

 variable character. 



Locality and horizon. — "Isotelus Zone" near the base of the Lower 

 Maquoketa beds near where Otter Creek empties into Turkey River at 

 Elgin, Iowa, and at Clermont, Iowa. 



Genus MEGALASPIS Angelin 1878. 



Cephalon having its anterior portion large and flattened. Glabella 

 short, more or less prominent, in front of which the facial sutures unite, 

 usually, in a long drawn out point. Doublure of the cephalon divided 

 by a median suture. Hypostoma arched, not forked, emarginate or 

 drawn out in a point posteriorly. Thoracic axis small, pleurae rounded 

 at the ends. Pleura? of the pygidium grooved, doublure of the pygidium 

 narrow with a channel-like excavation. Range Ordovician, Europe and 

 North America. 



This generic description is adapted from Schmidt's* discussion of the 

 Asaphidas. Beecherf makes the presence of a well-defined, cylindrical 

 glabella the distinguishing feature of the genus, but a study of the 

 various European species discloses the fact that the form and definition 

 of the glabella are variable characters. 



Megalaspis beckeri J sp. nov. Plate XIV, Fig. 5. 



Type specimen in the collection of Mr. A. G. Becker. 



General outline of the test subelliptical with its anterior margin 

 produced into a long acuminate process, trilobation only moderately 

 well developed. Entire surface appears smooth to the unaided eye, 

 but under a lens, minute, rounded pustules are visible scattered over 

 the surface. These pustules are larger and nearer together in the axial 

 region, especially just in front of the glabella. 



Cephalon subtriangular in outline, with all sides of the triangle 

 concave, marginal border narrow and not well defined. The posterior 

 portion of the cephalon is convex, the anterior portion flattened and 



*Rev. Ostbaltischen Sil. Trilobiten Abt. V, lief. 1 and 4. 

 fZittel-Eastman Text Book Pal., p. 630 



JThis name was proposed by Mr. E. O. Ulrich and the figure is from a photo- 

 graph furnished by him. 



