152 GEOLOGICAL SURVEY OF THE TERRITORIES. 
flattened on the outer side, rather Teor and located about half 
way between the middle and the anterior end; anterior umbonal slopes 
prominently rounded, or forming a rounded undefined ridge, which de- 
secends nearly vertically from the anterior side of each beak to the 
antero-basal margin; the sides behind this ridge being a little flattened, 
or possibly sometimes slightly concave below. Surface marked by 
small, rather regular, but not strongly defined, concentric ridges that 
become nearly obsolete on the posterior dorsal region and near the 
front. 
‘Length, 1.27 inches; height, 0.92 inch; convexity, 0.66 inch. 
“This shell closely resembles some varieties of Pleuromya jerruginea 
and P. impressa Agassiz, but has the anterior end shorter and more 
truncated, the concentric ridges of less regularity, and the concavity 
extending from the beaks to “the anterior basal margins of the valves 
either entirely wanting or very feebly marked.” 
Position and locality.—This species has a wide geographical range in 
the Rocky Mountain region, it having been recognized in Jurassic strata 
of Colorado, Utah, Wyoming , Dakota, Montana, and Idaho. The spe- 
cimens figured by Meek came from Utah, while those figured on plate 
38 are from Montana, mostly from near the Lower Caton of Yellow- 
stone River. 
Genus LYOSOMA White. 
Etym: Avo, to loosen, and céua, the body. 
Shell resembling certain forms of Neritina and Nerita in general as- 
pect ; volutions few, the last one much expanded; outer lip moderately 
thin; inner lip not thickened and apparently without any callus; the 
portion of the body, exclusive of the last volution, very small and with. | 
out a proper columella. Both of the only two species yet known havea 
slight flattening or lessening of the convexity of both the upper and 
outer sides of the last volution; the upper side having a more or less 
distinct but shallow revolving depression along its middle portion. 
The only two species which are yet assigned to this genus are the 
Neritina? phaseolaris and Neritina ?? powelli of White, both of which 
are from the Jurassie strata of Utah. ‘The family relations of this genus 
are doubtful, but they are probably with the Velutinidw. They are ap- 
parently both marine forms; but in view of the fact that several species 
of fresh-water mollusea have be en found in the Jurassic strata of West- 
ern North America, it is proper to state that the forms here under dis- 
cussion have considerable similarity to Pompholyx Lea, a fresh water 
genus. 
Lyosoma phaseolaris was described and figured in vol. iv, Expl. & Sur. 
West of the 100th Merid., p. 167, pl. xiii, fies 1a, b, ¢, a, and é, published 
in 1876. At that time I had not fully determined’ the character of the 
inner lip, but I have since become satisfied that it has no such thicken- 
ing as characterizes the Neritide. LD. powelli was described in Powell’s 
Report on the Geology of the Uinta Mountains, page 110; and although 
I then ascertained beyond doubt that no thickening of the inner lip ex- 
ists upon any of the specimens that had been collected up to that time, 
T hesitated then to do more than suggest for that form provisionally 
the generic name Lyosoma. This hesitation was caused by the publica- 
tion “ot Binkhorst’s discovery that the callus of the inner lip of certain 
fossil species of Nerita has been dissolved away during the condition of 
its fossilization, leaving the substance of all the remainder ot the shell 
