18 THE MOLL USC A OF MUSCATINE COUNTY. 



or less distinctly ra}~ed : mature specimens are generally dark 

 or black over the umbones, though some are decidedly yellow. 

 Nacre generally white, often rose and salmon. Some forms 

 resemble ellipsis (Lea) ; are figured and described in " Mono- 

 graph of the Unionidse," by Conrad, under the name of crassus 

 (Say) ; and in Lea's " Observations on the Genus Unio," vol. 

 ix., it is called higginsii (Lea). The figures and descriptions 

 of higginsii were made from specimens obtained at Muscatine 

 by Mr. Frank Higgins. Some of the shells before us are un- 

 doubtedly higginsii. In " American Concholog}^ " Sa} T figures 

 and describes another form of this species as abruptus. Out 

 of twenty-one examples now before us, eight are certainly 

 abruptus of Say. Lea places abruptus and crassus as s}~no- 

 nymes of orbiculatus in his u Synopsis of the Family Unioni- 

 dse " (1870) ; and we are constrained to add higginsii to the 

 same list. The difference in form is, no doubt, caused by fer- 

 tility or sterility, and kind and quantity of food. 



U. parvus, BARNES. Hardly common. Mississippi, Muscatine 

 Slough, Cedar, Mud Creek, Des Moines, etc. Length, 3.8 ; 

 breadth, 2 ; diam., 1.4 cm. 



U, plicatus (LESUEUR?). Abundant. Mississippi, Muscatine 

 Slough, Cedar. Some doubt exists as to the separation of 

 this species from undulatus of Barnes, or costatus of Raf. 

 The figure and description of costatus in Conrad's "Mono- 

 graph," plate VII., agrees with a form found in Mud Creek 

 at Wilton, and in the Des Moines at Des Moines, which we 

 call undulatus; and plate LIX. in same work is our plicatus. 

 Plicatus, as we have it, is a very heavy shell, reaching one 

 pound four ounces, which is more than three times the weight 

 of our largest and heaviest undulatus from Mud Creek. Pli- 

 catus is generally very tumid, has high and prominent umbones, 

 and the epidermis of very young shells is decidedly green. 

 Undulatus is much less tumid, very low umbones, and young 

 shells from Des Moines (the young of this species have not 

 been found in this count}') have a light-j'ellow epidermis. 

 Length, 14.5; breadth, 10.5; diam., 7.6 cm. 



TJ, pustulatus, LEA. Rare. Mississippi. Length, 7.2 ; breadth, 

 6 ; diam., 4.4 cm. 



U. pustulosus, LEA. Mississippi, Cedar, and Des Moines. We 



