6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



spines observed. Thorax distinct because of the loose arrangement of 

 the segments. Hypostoma unknown. 



Callazna. — Glabella fails to reach the rim, but there is no median 

 ridge. Rim wide but poorly defined, in fact it may be said to be 

 lacking. Intergenal spines very strong. Well developed occipital and 

 thoracic spines. Third thoracic pleura not enlarged. No strong spine 

 on the fifteenth segment. Hypostoma without posterior spines, 

 attached directly to a broad sickle-shaped plate. This plate separates 

 from the cephalon along a marginal suture (intramarginal) that corre- 

 sponds to the one on the inner edge of the rim in Mesonacis, and it 

 is divided into two similarly shaped pieces by the true marginal suture. 

 Thus both the marginal and epistomal plates are situated on the under 

 side. Since the combined width of the two plates is considerable they 

 bridge the space between the anterior margin and the glabella, so that 

 the hypostoma needs no stalk and hence there is no median ridge on 

 the top side of the cephalon. 



The other genera assigned to this family were not studied in this 

 connection and will be discussed in a later revision. 



FOSSILS FROM THE LOWER SHALE 



The following six species all occur in association in a fine-grained, 

 brown, somewhat calcareous shale, which is practically indistinguish- 

 able from the shales in both the eastern and western United States 

 that carry the same genera of trilobites. 



Since all the fossils come from one locality and occur in two beds 

 only, no locality or horizon will be listed following the descriptions. 



PATERINA PROSPECTENSIS Walcott 



Plate I, figs. I, 2 



Micromitra (Paterina) prospcctcnsis Walcott, 1912, Monogr. U. S. Geol. 

 Surv., No. 51, p. 352, pi. 2, fig. 4. 



The few specimens of this brachiopod from Bristol Mountain are 

 somewhat larger than the individuals from the type locality in Nevada, 

 but otherwise seem to agree with them in all respects. 



MESONACIS FREMONTI (Walcott), Restricted 



Plate I, figs. 3-9; plate 2, fig. 9; plate 3, fig. 8 



Olenellus fremonti Walcott (pars), 1910, Smithsonian Misc. Coll., Vol. 53, 

 No. 6, p. 320, pi. 37, figs. I, 2. 



A number of species are certainly included among Walcott's speci- 

 mens grouped under this specific name. The form from southern 



