I()4. 



Fresh Pond and its environs, at all times of the year, in company 

 with the C. truncata Linsley, and C. securis Nobis. Cabinet of 

 the Boston Society of Natural History. 



PisiDiuM coMPRESSUM. P. tcsta minuta, compressa, perobli- 

 qua, triangulari, postice abrupta, alba, nitida, umbonibus parvis, 

 apiculatis, non approximatis. Long. 0.16 ; lat. 0.14 ; diam. 0.9 

 inches. Hah. Massachusetts. 



Shell small, oblique, compressed, posteriorly abrupt, very 

 finely striated ; color very light gray with a slight and indistinct 

 zone of yellow on the margins ; beaks small, very prominent, 

 but not approximate. 



Remarks. This species bears some resemblance to the P. 

 variahile Nobis, but it is more compressed ; the beaks are 

 smaller, and it is in general smaller ; the epidermis is also dif- 

 ferent. Found, sparingly, at Fresh Pond, near Cambridge, in 

 company with the P. variahile Nobis. 



PisiDiuM ROTUNDATUM. P. tcsta minuta, subglobosa, tumida, 

 postice subtruncata, alba, nitida, tenui, tenuiter striata, umboni- 

 bus niagnis, tumidis, rotundatis, approximatis. Long. 0.95 ; lat. 

 0.75 ; diam. 0.08 inches. Hah. Lake Superior. (Agassiz.) 



Shell small, somewhat globose, tumid, anterior and inferior 

 margins rounded, posterior somewhat abrupt; beaks very large, 

 tumid, rounded, approximate, removed slightly from the centre, 

 and inclining towards the posterior, surface smooth, striee very 

 minute ; color yellow, somewhat darker on the beaks. 



Remarks. This very small species bears some resemblance 

 to the P. ventricosum Nobis, but is less inflated, more equilateral, 

 the margins are more rounded, and the beaks are larger and 

 more tumid. 



General remarks. I have observed in the neighborhood of 

 Fresh Pond three species of Cyclas, hitherto foreign to this 

 State ; the C. truncata of Linsley, first found in Connecticut, and 

 the C. minor and C nitida of Adams, first found in Vermont. 



The number of the species of Cycladidcc in Massachusetts 

 amounts now to sixteen, ten of which belong to the genus Cyclas 

 and the other six to the genus Pisidium. They are : — 



Cyclas cardissa Nobis. 

 Cyrlas ccprvlen. Nobis. 



