PLATE IV.

Each conventional sign represents a shell. The lengths are found by reading from below upwards; the breadths, by reading from left to right. The marginal numbers represent millimeters.

The average of the given series is represented by the closed conventional sign.

DIAGRAM 1.

Based upon Table X, page 34. Limnophysa palustris Müll.

The circles represent specimens from Honey Lake, California; the squares, shells from Warm Spring Lake, Utah; and the triangles, specimens from brackish springs, Promontory, Utah.

DIAGRAM 2.

Based upon Table XI, pages 34 and 35. Physa gyrina Say.

The circles represent shells from brackish springs, Promontory, Utah; the squares, specimens from fresh-water ponds at Salt Lake City, Utah.

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PLATE V.

Each conventional sign represents a shell. The lengths are found by reading from below upwards; the breadths, by reading from left to right. The marginal numbers represent millimeters.

The average of the given series is represented by the closed conventional sign.

DIAGRAM 1.

Based upon Table XIV, page 39. Physa ampullacea Gould.

The circles represent shells from Little Gull Lake, California; the squares represent shells from Church Lake, near Salt Lake City, Utah.

DIAGRAM 2.

Based upon Table XVI, page 41. Fluminicola fusca Hald.

The circles represent fossils from Kelton, Utah; the squares, shells from Ogden River; and the triangles, specimens from Utah Lake, Utah.