55 



species from the Ridean. They are probably due to injuries received when young 

 through comius? into violent contact with a rock or pebble. To such a mishap 

 the young of this species must often be exi)oeed in the rapid water they frequent. 



Genus Anodonta, Bruguihres. 



The transition from Margaritana to Anodonta is by no means abrupt : nihil in 

 natura ppr saltum. It is made easy by a shell found here, which was first 

 described by Say, and placed by him in the former genus — or rather in the genus 

 corresponding to it that he had instituted, ala-wiodonta, — but which is at present 

 universally referred to the latter. This species is now known as Anodonta edentula, 

 Say. Although its name as it now stands expresses what may be called the 

 reduplication of toothlessness, the shell is slightly exceptional to the best marked 

 character of the genus — the absence of both cardinal and lateral t'leth. 



Anodonta edentula, <Say, like its relatives the mar g aritanie^ i& to hi found in 

 water flowing rapidly over a rocky bottom. The best localities along the Ottawa 

 that I have met with are the Little Chaudiere and Chats Eapids. A capital place 

 for collecting it and seven or eight other species of the Unionidm is the snye, as 

 the lumbermen call it, between Mason's Mill and the opposite island. It is a 

 comparatively thick shell, generally of a dark olive colour ; but when the rays 

 are few or narrow, the ground tint, a light brown, predominates. In the left valve 

 of many specimens there is a short though well defined cardinal tooth with a 

 small notch in it analogous to the deep cleft in the primary tooth of the left 

 valve of Unio and 3Iargaritana. 



In the narrowest and most rapid parts of Meech's Creek, and not in the 

 ponds into which it otten expands, or tlie lake from which it flows, there occurs a 

 fine form of this shell which appears to be identical with the variety of ^. edenlula 

 described by DeKay, and called by him, after the river in New York in which 

 it is found, A. Unadilla. It is more inflated than the A. edentula from the Ottawa, 

 often very much larger and of a lighter colour. 



Anodonta undulaia, Sag, is found in the Rideau near Billings' Bridge, and in 

 the Ottawa at Kettle Island. It resembles the preceding species so much that 

 many have thought the two identical. A. tnidulata is however a thinner shell 

 more ob.scurely rayed and more angularly inflated. Additional and f^r more 

 distinctive characters are revealed by the microscopic examination of the young 

 of both species. Botanists, as Mr Fletcher told us two years ago, cannot always 

 by the leaves and blossoms alone distinguish Drosera longifolia from Drosera 

 rotundifolia, but their minute seeds present characteristics which place the speci- 

 fic distinctness of the parent plants beyond all doubt. So also with the embryonic 

 young of these two species of anodonta. I have not examined them myself; but 

 Dr. Lea's figures show that tliey differ in outline, and that while the hooks of A. 

 ■ edentula end in three points, those of A. undulata end in one. 



Anodonta subcylindracea, Lea, which I have met with only at the Chats, is 

 one of the most widely distributed shells of the genus, extending hence through 

 the middle and western states as far south as Louisiana. Our shell in its ordi- 

 nary form is identical with Dr. Lea's type. It is small, thin, inflated, almost 

 elliptical in outline, and olive green in colour, with indistinct rays. Old shells 

 are generally abnormal. They are so constricted along the basal margin oppo- 

 site the hinfje, and so much elongated that instead of being elliptical they are 

 kidney shaped. This reniform appearance is observable in old shells of many 

 species of the Unionidie, U. complanatus, for instance, and notably M. margaritifcra. 

 An examination of the lines of growth will show that after a certain age the 

 shell does not increase symmetrically. It grows rapidly in the direction of the 

 umbonial slope, slowly in front, and scarcely at all opposite the hinge. The 

 change produced in this way in the form of shells is very remarkable. 



Anodonta Benedictii, L'a, occurs in several localities near the oitv, but no- 

 where iu great numbers. I have found it at the Chats, and in a sm dl lake on 



