'7 



The Nautilus. 



Vol. XXIII. JUNE, 1909. No. 2. 



SOME OBSERVATIONS AND NOTES ON MUSCULIUM. 



BY V. STERKI. 



Most or all of our species of Musculium Link ( Calyculina Clessin) 

 are very variable. E. g., of M. securis Pr. there are almost endless 

 forms, some of them so different from others that they appear to be 

 distinct species, even of different groups. They readily respond to 

 the nature of their habitats, and almost every place has its own pecu- 

 liar forms. It may be added that most of them are inhabitants of 

 quiet waters: small lakes, ponds, pools, marshes, ditches, slow 

 streams ; but M. transversum Say is also found in rivers with strong 

 current, with stony and rocky bottoms. 



Years ago Clessin stated his belief that they are comparatively 

 short-lived and of cyclical development, annuals. The first part of 

 his statement is probably correct, the latter probably not, or not for 

 all species; specimens at all stages of growth, from newly hatched to 

 full-grown, can be found at any time of the year. Yet under certain 

 conditions their development appears to be uniform; e. g., where 

 pools dry up in fall, only the young mussels appear to survive, to 

 grow to maturity and propagate during spring and summer. 



These mussels are described as having their beaks calyculate, or 

 " capped," and the genus has been established mainly on that fea- 

 ture.* But in most and probably in all species, specimens and forms 

 are found with slightly or non-calyculate beaks, and such are the rule 



* Yet even without that supposed but mistaken character the genus appears 

 to be well founded, as will be shown elsewhere. 



