126 (Dec. 



from an alternation of contraction and dilatation, corresponding usually to ten 

 segments, into which the animal may subdivide, but more or less disappearing in 

 elongation of the body, becoming more apparent by wrinkling in shortening of 

 the body ; anteriorly semi-ovate, sub-acute ; posteriorly elongated, cylindroid, 

 obtusely rounded. Apparent segments panduriform, furnished each posteriorly 

 with a clear globular nucleolated nucleus. Intestine variable in capacity, usu- 

 ally dilated in the anterior dilatation of each apparent segment, and much con- 

 tracted in the intervals. 



Length from 1-2 to 2 lines, shortening to 1 or 1-4 aline ; breadth, when elonga- 

 ted, from l-400th to 1 300th in. ; when shortened from l-300th in. to 1-2 of a 

 line. 



Habitatio7i and Remarks. Found in the same situation as the preceding, 

 creeping planaria-like upon different substances, or most frequently holding 

 a vertical position in the water, apparently without movement, but re- 

 taining their position by means of the actively moving vibrillae, whicii are com- 

 paratively larger than in the preceding worms. They appear to feed upon vegetable 

 particles brought to the mouth by means of the currents produced by vibrillae. The 

 intestine is usually empty, except at the dilated portions, where it is yellowish or 

 greenish, from granular matters contained within. The whole structure of the 

 animal is exceedingly simple, composed of nucleolated, granular corpuscles, those 

 forming the exterior of the body being furnished with vibrillee. Under slight 

 pressure, these corpuscles undergo separation from one another, and become 

 globular by endosmosis; in this state they measure from the l-7000th in. to the 

 l-2800th in. The nucleoli are globular, shining, and measure the l-90Qthin. in 

 diameter. The exterior vibrillated corpuscles, after separation from the body, 

 often move about for some seconds. The vibrillee measure about l-3500th in. 

 long. Each segment of the animal's body, at its posterior part, contains a globu- 

 lar, transparent nucleus, measuring the l-2333d in. in diameter, with a globular, 

 refractile nucleolus the l-7000th in. in diameter. This latter body, with the form 

 of the apparent segments, makes the animal resemble a row of gregarinae attached 

 together. 



The Committee on the following paper by Mr. Cassin, read Dec. 

 24th, 1850, reported in favor of publication in the Proceedings. 



Notice of an American species of Duck, hitherto regarded as identical with the 



Oidtmia fiisca, {Linn.) 

 By John Cassin. 



Oidemia velvetina, nobis. Audubon, Birds of Am. pi. 247. 



Form. 5 Generally similar to that of 0. fusca, but is rather larger, bill 

 longer and more slender, with the protuberance at the base of the upper mandible 

 more prominent and more extended. Space between the nostril and the edge of 

 the upper mandible much narrower in the present species. 



Upon the protuberance at the base of the bill in this species, the velvet like 

 plumage of the front terminates in a well defined point, as represented in the 

 figure of the female in Aud. pi. 247, while in 0. fusca this termination is abruptly 

 sub-rounded as in both figures in Selby, Ill.pl. 67, though I have never seen a 

 specimen in which this rounded character is so distinctly marked as there 

 represented. 



