ORTMANN: monograph of the naiades of PENNSYLVANIA. o3 



trapezoidal, or almost rectangular, considerabl}' longer than high. Beaks moder- 

 ately prominent. Beak-sculpture consisting of a number of bars, the first of which 

 seems to be simple, the following two or three distinctly- double-looped, with the 

 posterior loop angular and nodose upon the posterior ridge. Farther on, upon 

 the disk, the nodes of the posterior loop are continued a^s strong tubercles, while 

 the anterior loop breaks up into small granules, which generally assume a zig- 

 zag arrangement. They are continued downward to a variable extent, but gen- 

 erally they soon disappear. Shell greatly swollen, from almost subcylindrical to 

 rather flat, with a broad and distinct posterior ridge. In front of the ridge the 

 shell is flattened, but has no radial furrow; behind the ridge the shell is more or 

 less depressed, thus generally producing a slight emargination of the posterior 

 margin. Upon the posterior ridge stands a row of tubercles (continuing the beak- 

 sculpture), which increase in size toward the posterior angle, and may be larger 

 or smaller; in some cases they are more or less obliterated. In front of the pos- 

 terior ridge the shell is generally smooth, but towards the beaks we find a number 

 of small tubercles or nodules, described above as a continuation of the broken-up 

 anterior loops of the beak-sculpture. The development of these nodules is quite 

 variable. The posterior slope may be ornamented with ridges or radiating rows 

 of nodules, or may be entirely smooth. Epidermis 3'ellowish to browTiish or 

 greenish, with dark green rays broken up into a characteristic pattern of triangular 

 spots, pointed toward the lower margin. The distribution and size of these spots 

 is rather irregular and variable, but they are generally well-developed and distinct, 

 and only in rare cases do they become obliterated. Linear rays are, as a rule, 

 entirely absent, indications of them are seldom seen upon the posterior ridge or 

 posterior slope. 



Hinge-teeth well-developed, but not very heavy. Pseudocardinals divergent, 

 ragged. Interdentum narrow or almost absent. Lateral teeth long. Beak- 

 cavity moderately deep. Dorsal muscle-scars on the hinge-plate. Nacre alwaj'S 

 ■ white. 



Sexes absolutely indistinguishable in the shell. 



L. H. D. 



Size: 1. Utica, Cat. No. 61.38.52 (without any nodes) .... 123 mm. 50 mm. 45 mm. 



2. MeadvUle, Cat. No. 61..3S50( without any nodes). 119 " 5-1 " 36 " 



3. Godfrey, Cat. No. 61.4358 (with strong nodes) . . S3 " 33 " 33 " 



4. Aladdin, Cat. No. 61.3856 (with strong nodes) . . 84 " 37 " 30 " 



Pennsylvanian specimens compare favorabh- with previous measurements; in 

 fact, the size of the first one given represents the maximum on record. 



Soft parts (See Ortmann, 1912, p. 256). It is to be added that gravid females 



