﻿44 DESCRIPTIONS OF THE EMBRYONIC FORMS OF 



Having carefully examined the soft parts of many thousand specimens, living and 

 preserved in alcohol, in every case I gave close attention to the condition of the 

 female ; and when I found the ova so much advanced as to give a determined form to 

 the embryonic shell, I made enlarged drawings and careful descriptions. I soon 

 found that these singular forms, so different from the adults of the same species, 

 which are usually transversely oval, naturally arranged themselves into groups 

 having outlines entirely different. Thus there were pouch-shape, subrotund, subtrian- 

 gular and wedge shape outlines, the dorsal line being straight, or nearly so, and con- 

 necting the two valves the whole length of this margin. They seemed to be in no 

 way analogous to the mature parent, except that they possessed two valves. In the 

 plane of the outline there was no approximation in form. The base, in all the species, 

 was either angular or rounded, and always presented the anterior and posterior mar- 

 gins equal, which is not the case with any of the species when fully grown. That 

 is if a perpendicular line be raised from the middle of the basal margin to the middle 

 of the dorsal line, the right and the left divisions will be exactly symmetrical. 

 Most of the Margaritance and Anodontce have a remarkable hook or spur on each 

 valve, which, when the valves are closed, both these hooks are enclosed, and seem 

 to be used for keeping the valves together. This hook is placed at the base or angle, 

 directly opposed to the middle of the dorsal line ; I have never observed this hook, 

 which is always more or less granular or serrate on the outer side, except in the 

 species which are, in this stage, sub-angular at the base. The species which are round 

 or much curved at the base, never have it, so far as my observation has extended. 



When the young are still within the branchial uterus,* but ready to come forth, their 

 epidermis gives a color to it, and the young shell itself, when taken out, exhibits this 

 color. In the Anodontce and Margaritance, they are usually, but not always, of a light 

 brown tint ; some are whitish. The color of the branchial uterus of the Unio is 

 usually white, and the epidermis of the young shell also. But this is not always so, 

 for in some species they are found quite brown, as in the case of Unio irroratusf (no- 

 bis,) and some others. 



Under the microscope, with a high power, the whole exterior surface of the valve, 

 when taken out of the branchial uterus, will be found to be granulate. This condition 

 is invariable in all the species of this family, so far as I have been able to observe 

 them. In some cases these granules are closer and larger than in others, but they 

 are exceedingly small; those of Margaritana rugosa being 0.0045 of a millimeter 

 in diameter. They are attempted to be represented in the plate. The cluster of 

 spots which is observed in nearly all the drawings, is not occasioned by these granules, 



* There are some species which extrude the whole sack before the young are perfect in the ova. I mentioned 

 this in Trans. Am. Phil. Soc, July, 1836, vol. vi„ N. S. 



t See Tr. Am. Phil. Soc, vol. iii., pi. 5, and Observations on the Genus Unio, vol. i., p. 11. 



