66 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 75 



pronounced longitudinal keel. Fixed cheeks wide, with strong ocular 

 ridges crossing them. Palpebral lobes rather small and slightly up- 

 turned. The broad frontal limb and border in front of the eye-lines 

 is marked by irregular inosculating lines. There is a tendency toward 

 the formation of a boss immediately in front of the glabella, a feature 

 which seems to occur in a greater or less degree in nearly all trilobites 

 with a wide frontal limb. The present incomplete study indicates a 

 possibility that several of the American species now referred to 

 the European genus Acrocephalites may ultimately be included in 

 Amecephalus. 



The free cheeks are of moderate size and of the usual shape. They 

 have a very narrow border with a tendency to turn up somewhat into 

 a wire edge. A comparatively wide doublure is present under the 

 cheeks and possibly maintains its width across beneath the cranidium. 

 The facial suture is intramarginal for about one-third the distance in 

 front of the cranidium. The broad frontal border has a very narrow 

 rim which is usually not apparent in the flattened specimens. 



The thorax in the type species has 19 segments. Those toward the 

 rear of the body have relatively longer spines that partly envelop the 

 small pygidium. 



Pygidium small, smooth, and definitely three-lobed. The axial lobe 

 is considerably larger than the side lobes and extends to the posterior 

 margin ; it has two or three transverse furrows that do not appear to 

 extend out on the pleural lobes. 



Derivation of name. — Ajut^ = shovel or spade; K£<^aAr;=head. 



Genotype. — Ptychoparia piochensis Walcott, as restricted. 



Range. — Middle Cambrian of the Great Basin, Nevada, etc. 



AMECEPHALUS PIOCHENSIS (Walcott) 



Plate IS, figs. 8-10 



Ptychoparia piochensis Walcott, i885, U. S. Geol. Surv. Bull. 30, p. 201, pi. 

 26, figs. 2, 2a, b; pi. 28, figs, i, 2-2e. (Described and illustrated.) Pack, 

 1906, Journ. Geo!., Vol. 14, p. 297, pi. 2, figs. 4-4C. (Added notes and 

 illustrated.) Grabau and Shimer, 1910. N. A. Index Fossils, Vol. 2, 

 p. 276, fig. 1575. (Illustrated.) 



Liostracus piochensis Lorenz, 1926, Zeits. d. d. geol. Gesell., bd. 58, heft i, 

 p. 61. (New generic reference with notes.) 



Amecephalus piochensis Walcott, 1924, Smithsonian Misc. Coll., Vol. 75, 

 No. 2, p. 54. Pl- 9, %• I. 



Observations. — The original description includes the forms with 

 an extra wide frontal border and 19 segments which here are restricted 

 to this specific name. The specimens included under this generic and 



