34 ILLINOIS BIOLOGICAL MONOGRAPHS [132 



blister formation near the anterior and posterior end, one in each end of 

 two specimens. 



6. Amblema undulata (Barnes). Blue-Point; Three-Ridge. 



This characteristic mussel is the most abundant species in the Vermilion 

 River, greatly exceeding (with the possible exception of Lasmigona com- 

 planata) in number of individuals all other species. It is also able to resist 

 much of the ill effects of sewage pollution and is the first shell met with in 

 the polluted waters of the Salt Fork. Living specimens, however, were 

 not seen above the station called bench mark 655, a distance of over 15 

 miles from the source of pollution. Empty shells and odd valves occur 

 more or less abundantly from St. Joseph, 10 miles below Urbana, to the 

 station mentioned. As the species lives in fair abundance in the tributary 

 known as Spoon River, for a distance of over two miles up stream from near 

 the mouth of the stream, it is evident that at one time its distribution was 

 equally continuous in the Salt Fork below the junction of Spoon River 

 with Salt Fork, where now there is a break of nearly six miles. This break 

 in the distribution is in all probability due to the sewage pollution, for the 

 stream is admirably adapted by nature as a habitat for this species and 

 has not been disturbed by dredging. 



There is great variation in the form of the shell. Many specimens from 

 the upper part of the stream, both Spoon River and Salt Fork as far down 

 as the natural dam, are almost round with a broad 'wing' above the undula- 

 tions, which may be reduced in number and form (Fig. 34). These 

 shells may be inflated or rather compressed. Other shells are more quad- 

 rate and in occasional individuals the umbones are elevated simulating 

 Amblema peruviana {plicata of authors). These shells have a black or 

 dark brown epidermis in the adult condition. In Spoon River young 

 shells 25 mm. long were common, but few young specimens were found in 

 the Salt Fork above the Homer Park dam. Shells from the lower part of 

 Salt Fork, below Sidney, are as a rule cleaner, the epidermis is of a brighter, 

 lighter brown and are more uniformly quadrate than those from above 

 Sidney. The largest specimen collected measured 140 mm. in length and 

 this seems to be the maximum size for the undulata in this stream. Many 

 of this size were seen. 



In the Spoon River, and in the upper part of Salt Fork, injured shells 

 are common. The injuries consist of breaks in the shells and subsequent 

 repairs. In one specimen from the upper part of Spoon River, an injury 

 had been received when the mussel was small which resulted in a deep 

 channel across the right valve (Fig. 29) and a ridge, also slightly chan- 

 nelled, on the left valve (Fig. 30). Another shell had nearly a hundred 

 blister pearls on the edge of the posterior margin of the left valve (Fig. 28) 

 and a large blister pearl about midway of the pallial line in the right valve 



