658 CAMBRIAN BRACHIOPODA. 



the visceral cavity in front of the opening is short, and in some shells very minute pits along 

 the front indicate muscle scars that are too imperfectly preserved to be identified. 



In the posterior half of the interior of the dorsal valve there is a strong median ridge that 

 bifurcates near the center; this ridge enlarges where the central muscle scars (h) occur on its 

 outer slopes and contracts before bifurcating just in advance of the anterior lateral muscle scais 

 (j) (PL LX, fig. 1 1) . The main vascular sinuses originate near the posterior margin at the end 

 of the median ridge and extend first outward and then obhquely forward as narrow, nearly 

 straight sinuses that are most deeply impressed in the shell at the inner and outer margins. 

 The muscle scars have left very faint impressions on the shell. The cardinal scars (cl) are 

 small and situated at the sharp angle formed by the main vascular sinuses and the posterior 

 margin of the shell (PI. LX, fig. Ik). The size and position of the central scars (h) and anterior 

 lateral scars are indicated on Plate LX, figure 1 1. 



I have been able to discover no trace of a true area in either valve; the posterior margins 

 of the valves show concentric strise and often the under side is striated, the surface apparently 

 not having been rubbed against the edge of the opposite valve. 



Observations. — There are some noticeable variations in this species, one of which I have 

 separated as the variety Isevis. In addition there is a variation in the position of the apex 

 of the ventral valve that is often very marked; part of this variation is explained by the com- 

 pression and consequent distortion of the shells, but there is also a range in the distance of 

 the apex from the posterior margin of about one-fifth to one-third the length of the valve. 



Acroihele suhsidua Isevis is separated on accoimt of the smoother surface of a large group 

 of shells occurring in the Middle Cambrian limestone near Ophir, Utah. 



The specimens from the Ijower Cambrian of Pioche, Nevada, which I [1886b, p. 109] 

 referred to Acroihele suhsidua are now referred to A. spurri, as more perfect specimens show 

 that although they liave the same type of surface the apex of the ventral valve is nearer the 

 posterior margin and the posterior margin is strongly arched below the false area. 



A. suhsidua is the Cordilleran representative of the Swedish Acroihele coriacea linnarsson 

 and the American Atlantic coast A. matihewi (Hartt). It differs from A. coriacea in its more 

 finely granulated surface, in the average more anterior position of the apex, in the average 

 more circular outline of the ventral valve, and in the main vascular sinuses and their branches 

 in the ventral valve. From A. matihewi it differs in sm-face characters and interior vascular 

 markings. A. suhsidua is much like Acroihele {RedlicheTla) granulata- (Linnarsson) in form 

 and size, but it differs in not having the strong granulations of the latter species upon its 

 outer sm-face, also in haviag different interior vascular markings. 



The vertical range of this species in the House Range section is from the lower portion 

 of the Wheeler formation up iato the top of the Marjum limestone, a vertical distance of about 

 1,200 feet. In the shales of the Wheeler formation it is associated with Asaphiscus wheeleri 

 Meek, Ptychofaria Tcingi (Meek), and Agnostus interstrictus White. In the Marjum limestone 

 this famia disappears and a new subfaixna of the Middle Cambrian fatma appears in which 

 Oholus mcconneili (Walcott) and Micromitra (IphideUa) pannula ophirensis (Walcott) occur. 

 At Ophir, in the Oquirrh Range, Acroihele suhsidua Isevis is abundant at about the same 

 horizon as the lower portion of the Marjum limestone. On the west front of the Wasatch 

 Range, in Utah, it is associated with Oholus mcconneUi (Walcott) and Acrotreta ophirensis 

 Walcott. 



Formation and locality. — Kiddle Cambrian : (55t) a About 350 feet (106.7 m.) above the Brigham quartzite, 

 in the limestone of the Ute limestone [Walcott, 1908a, p. 7], on the west side of the road, 0.5 mile (0.8 km.) above 

 the forks, Paradise Dry Canyon (locally known as East Fork), east of Paradise, Cache County, Utah. 



(32p and 65e) Spence shale member of the Ute limestone, about 100 feet (30.5 m.) above the Brigham quartzite 

 [Walcott, 1908f, p. 197], at the mouth of the first small canyon south of Wasatch Canyon; and (S4c[) a drift block sup- 

 posed to have come from about 1,700 feet (518.2 m.) above the Brigham quartzite, a horizon correlated with the shales 

 forming 2d of the Bloomington formation in Blacksmith Fork Canyon [Walcott, 1908f, p. 195], in Wasatch Canyon; 

 both east of Lakeview ranch, about 5 miles (8 km.) north of Brigham, Boxelder County, Utah. 



a The species is somewhat doubtfully identified from this locality. 



