114 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull. 106. 



fine, perfectly regular, concentric, thread-like lines, gradually becom- 

 ing smaller and more crowded toward the uinbones, on which they arc 

 nearly or quite obsolete. 



"Length, 0.90 inch; height, 0.G0 inch; convexity, about 0.20 inch." 

 Prof. Meek explains that the species is known only in the form of 

 sandstone casts that do not show the hinge nor any other internal char- 

 acters and that its reference to Tellina is very doubtful. It seems to 

 me more probable that it is a Mactra, but as I have not been able to 

 demonstrate the character of the hinge it is left under the original 

 name. 



Locality and position. — East Canyon creek, Wasatch range, Utah ; 

 sandstone of the u second ridge v at Coalville, Utah. A cast that 

 seems to belong to this species was found in the " third ridge " of the 

 same section. 



SOLENID^l. 



Genus SILTQUA Megerle v. Mnhlfeldt. 



SlLIQTJA HUERFANENSIS n. Sp. 



PL xxv, Figs. 10 and 11. 



Shell long and narrow, inequilateral, somewhat compressed, gaping 

 at both ends; dorsal margin very slightly convex; basal margin nearly 

 straight or slightly sinuous in the middle; both ends broadly rounded, 

 but the anterior end slightly narrower than the posterior; beaks very 

 small and inconspicuous, located a little in advance of the middle. 

 Substance of the shell thin, marked on the surface by very faint lines 

 of growth and by a few larger irregularly arranged concentric undula- 

 tions. 



Pallial sinus broad, rounded and rather deep. Muscular impres- 

 sions near the cardinal margin, well marked on adult shells; the ante- 

 rior one elongate and narrow, just in front of the beak; posterior im- 

 pression ovate, about half way between the beak and the posterior end. 

 An internal rib runs obliquely downward and forward from the beak, 

 gradually becoming less prominent and ending about half way between 

 the dorsal and ventral margins. The structure of the hinge has not 

 been satisfactorily made out, but some of the casts show impressions 

 of two or three very small cardinal teeth. 



Length of medium- sized specimen, 5S mm ; greatest height, 18 mm ; con- 

 vexity of the two valves united, about 9 mm . 



In outline this shell is very similar to some recent species of Pharella, 

 but the presence of the internal rib and the large size of the pallial 

 sinus will not allow a reference to that genus, and on account of these 

 features it is provisionally referred to Siliqua. I think that Pharella f 

 dakotensis M. & H., from the Dakota formation near the mouth of Big 



