172 MEMOIRS OF THE CARNEGIE MUSEUM. 



the characteristic fine sand of the peninsula, sometimes mixed with more or less 

 mud. 



This variety has been reported from other localities in Lake Erie, for instance 

 at Buffalo (Marshall, 1895), and also from Lake Michigan in Michigan (Walker, 

 1898) and Illinois (Baker, 1898a; Walker, 1913, p. 22). Records from other 

 larger or smaller lakes and from canals are abundant from eastern Canada, Lower 

 St. Lawrence region (according to Bell, 1859), New York (crossing over by way of 

 Erie Canal to the Hudson at Albany) (Marshall, 1895), northern Ohio (Sterki, 

 1907a), Michigan (Walker, and Baker, 1914), as far north as Schoolcraft and 

 Alger Cos., near Lake Superior (Winslow, 1917). But it also has been reported 

 from creeks and rivers, and its type-locality is a creek in Ohio. It is found in 

 New York, Ohio, Indiana (Call, 1896a, and 1900), and in Illinois (Baker, 1898a, 

 and 1906) in streams running to the lakes as weU as to the Ohio. But on the 

 whole its range is more northeastern, and centers mainly in the drainage of the 

 Great Lakes and the St. Lawrence. Southward and westward it does not go 

 beyond Ohio and Indiana, nor beyond the northeastern comer of Illinois. 



Genus Alasmidonta Say (1818). 



Ortmann, 1912, p. 294; Simpson, 1914, p. 492; Ortmann, 1914, p. 44. 



Type Monodonta U7idulata Say. 



This genus has been divided into subgenera by Simpson, but they have been 

 somewhat modified by the present writer. Three subgenera are found in Penn- 

 sylvania. 



Key to the Subgenera. 

 a I. Hinge complete, with pseudocardinals and laterals, but the latter reversed, two in right, one in left 



valve. Beak-sculpture moderately heavy Subgenus Prolasmidonla. 



a 2. Hinge incomplete, laterals obliterated, pseudocardinals present. Beak-sculpture very heavy. 



bi. Pseudocardinals well-developed, stumpy. Posterior ridge weak, posterior slope smooth (rarely 



with traces of wrinkles). Rays not broken up into spots Subgenus Alasmidonta. 



bi- Pseudocardinals weak, compressed. Posterior ridge distinct, posterior slope more or less cor- 

 rugated. Rays generally broken up into spots Subgenus Decurambis. 



Subgenus Prolasmidonta Ortmann (1914). 

 Ortmann, 1914, p. 44. 



Type Unio heterodon Lea. 

 There is only one species in this subgenus. 



