366 ROBERT TRACY JACKSON ON THE 



terial which was collected by that gentleman, from Mr. "William B. Mai'shall of the!New 

 York State Museum. The specimens embrace glochidial stages from the gill cavities of 

 the parent and adults with perfectly jji-eserved umbos, so that the early embryonic shell is 

 still intact, and thus affoi'ds the op])ortunity of a comparison with later stages. 



Unio cariosus, Say. The glochidinm stage of this species, fig. 44, p. 367, is sub-quad- 

 rate with a straight hinge line and rounded angles on the ventral border. The shell has a 

 double line around its border, the inner line representing the limits of an earlier stage of 

 growth. A similar double line exists in the glochidinm of all Unionidae observed, oi- which 

 I have seen figured, and Dr. Lea makes the same observation. On the middle of the ven- 

 tral border of the valves is a Vjery abbreviated spur which is not, howevei', spinous as in 

 Anodon. The single adductor muscle is situated near the hinge line and a little nearer 

 the anterior than the posterior border of the shell.^ The shell is granulated on its surface 

 and presents no traces of prismatic structure, the same feature being found in the glo- 

 chidinm of all the Unionidse observed. The glochidial shell of U. cariosus and other 

 species of the genus is whitish and much smaller than is the glochidinm in Margaritana 

 and Anodon. The glochidinm of Unio nasuhis, Say, is similar to U. cariosus as described. 

 Dr. Lea figures the glochidinm from the gills of the parent in twenty-five species of Unio. 

 Twenty are similar to U. cariosus in form with some little variation in contour. Of the 

 five remaining species three are sub-triangular resembling some species of Anodon; but 

 two have the ventral angles produced instead of rounded and each angle of either valve 

 beai-s a small spur. 



Unio pressus, Lea. A specimen 47 mm. in length has the umbos well preserved and 

 still bears the early glochidial shell. This embryonic shell is sub-triangular and has the 

 same form as is figured by Lea in a specimen from the gill chambers of the parent of this 

 species. It is immediately succeeded by the nepionie stage of the dissoconch which has 

 the prismatic structure and concentrically rugose growth which is characteristic of the 

 young shell in Unionidie and is often figui'cd at the umbos of well preserved specimens 

 (see Lea and other authors). The glochidial shell in its relation to the dissoconch there- 

 fore fills the requirements of the prodissoconch and may be compared with that stage as 

 described in many genera in this paper. The Unios above described wei'e collected in 

 Albany County, New York, by C. E. Beecher. 



In Margaritana undulata, Lea, the glochidinm, fig. 45, p. 867, is sub-triangular, but 

 the anterior end of the embryo is considerably the larger. The hinge line is straight and 

 the valves at this area are raised in little humps indicating abbreviated umbos, not shown 

 in the figure. The angle at the ventral border of the shell is quite shai'p;it is posterior 

 to a median line drawn perpendicularly thi'ough the shell and bears a denticulate spur on 

 either valve similarly to Anodon. In other features the shell is like the glochidial stage 

 of Unio. A young specimen of M. undulata 13 mm. long, fig. 48, has the umbos well 

 preserved and they retain the glochidinm or pi-odissoconch intact. The hinge line of the 

 prodissoconch is parallel to the hinge line of the succeeding dissoconch growth and the 

 larger end of the embiyonic shell is directed toward the anterior end of the shell. The 

 prodissoconch has the granulated surface characteristic of the glochidia of this and other 



' The glochidial shell of all or nearly all Unionida" is a perpendicularly through the hinge line and middle of the 



little larger on one side thau on the other of a line drawn ventral border of the valves. 



