RIVER SHELLS. 69 



ridges. Its color varies from white to black, but it is 

 usually of a yellowish brown. 



And now, since we have filled our lungs with fresh 

 sea air, and have collected Littorines to our heart's 

 content, we will vary our shell -gathering 

 journey a little, by taking a hasty trip 

 inland, and strolling along the rivers of Ore- 

 gon. Here we shall find abundant speci- 

 mens of the Plaited River shell, Gomobasis 

 plicifera^ Lea, Gon-i-ob^-a-sis pli-sif -e-ra. A 

 good illustration of this shell is shown in 

 Fig. 54. Fig. 54, in which you see the general shape, 

 and the average size of the shell. But there are 

 many varieties of this species, which vary slightly 

 from one another, yet which are very similar in all 

 essential particulars. Several of these varieties have 

 been described under different names, but this should 

 not puzzle the student. 



The shell of a perfect specimen is a long, slender 

 cone, though the first whorl or two may be missing, 

 and thus change the cone into a frustrum. The later 

 whorls are nearly smooth, but the earlier ones are 

 marked by folds, or plications, the presence of which 

 suggested the name plicifera. The aperture is ovate 

 and entire; the color of the shell is greenish black, 

 and its length is an inch or less. Many specimens 

 come from the vicinity of Salem, Oregon. 



Goiiiobasis nigrina^ Lea, ni-gri^-na. Fig. 55, is a 

 California shell, with numerous smooth, rounded 

 whorls. It is rather smaller than the last spe- 

 cies, but is of the same color. 



Goniobasis occata^ Hds., ok-ka'-ta, is from the 

 Fig.^5. San Joaquin river. In general form, as well as 

 in size and color, it resembles the preceding species, 



