46 DESCRIPTIONS OF 



and belongs to a very remarkable form of branchial uterus. The latter, from Ano- 

 donta xmdulata, of which I described and figured the branchial uterus elsewhere,* is 

 represented here five times the natural size. The sacks were extruded by the 

 parent from day to day, for about a month, in the middle of winter. Eight or ten 

 young were generally in each sack, but some sacks were so short as only to have room 

 for one or two ; these were marginal sacks. Immediately when the sacks came out 

 from between the valves of the parent, most of the young were seen to be attached 

 by the dorsal margin to the outer portion of the sack, as if it were a placenta. As 

 represented here, the valves were usually open, and the hooks could be observed on 

 every one of them. 



It may be observed by the plate, that the most common form of the embryonic 

 Unix) is pouch-shape. From Nos. 1 to 20 they are of that form. No. 21 is subrotund. 

 Nos. 22 and 23 are sub-triangular. Nos. 24 and 25 are wedge-shape. The most 

 common form of the Margaritance is sub-triangular. Nos. 2fi to 30 are of this form. 

 No. 31 is subrotund. As regards the Anodontce, those which I have observed in this 

 condition, seven species, Nos. 32 to 38, are allsubtri angular. 



On pi. 5 will be found the following genera and species, at the period of their ex- 

 trusion from the branchial uterus, when it is evident they are prepared to take care 

 of themselves. They are enormously magnified, the natural size being from ^ to 7^ 

 of an inch in length. The exact measurements of four species, Nos. 1, 21, 24 and 26, 

 are given below, where the diagnoses are given. 



I am greatly obliged to many friends for the pains taken to send me living speci- 

 mens, as well as those in alcohol, for the purpose of these examinations, and more 

 particularly so to Bishop Elliot, J. H. Couper, and J. C. Plant, of Georgia, Dr. Lewis, of 

 Mohawk, N. Y., J. Clark, of Cincinnati, H. Moores, of Columbus, Ohio, E. Billings, 

 Ottowa, C. W., and Dr. Shurtleff, Mass. 



Unio obttjsus, Lea, pi. 5, fig. 1. Pouch-shape ; dorsal line rather short; side margins flattened ; basal margin 

 gently curved ; color clear white. Has no hooks. f Chattahoochee River, Georgia. Length 0.256 milli- 

 metres. Breadth 0.192 mm. 



Unio Anodontoides, Lea, fig. 2. Pouch-shape; dorsal line rather long; side margins arcuate, sub-angular 

 on both sides at the base ; basal margin gently curved ; color clear white. Has no hooks. Cincin- 

 nati, Ohio. 



Unio mxjltiplicatus, Lea, fig. 3. Pouch-shape ; dorsal line long ; side margins slightly curved ; basal mar- 

 gin rounded ; color clear white. Has no hooks.J Cincinnati, Ohio. 



Unio rutilans, Lea, fig. 4. Pouch-shape ; dorsal line long ; side margins gently curved ; basal margin 

 slightly rounded ; color clear white. Has no hooks. Baldwin County, Georgia. 



* Trans. Am. Phil. Soc, vol. vi., p. 15, and Obs. on the Genus Unio, vol. ii. p. 52. 



t The descriptions of the branchial uterus will be found elsewhere in these papers, where the diagnoses of 

 the soft parts of these animals are given. The embryo did not appear to be perfect in this specimen. 

 X In this species the branchial uterus exists in allfoxw leaves of the branchiae. 



