﻿48 DESCRIPTIONS OF THE EMBRYONIC FORMS OF 



Unio triangularis, Bar., fig. 19. Pouch-shape, approaching to oval ; dorsal line rather long, obtuse at 

 the ends ; side margins arcuate ; basal margin rather suddenly rounded. Clear white. Has no hooks. 

 Part of the second valve is annexed. Columbus, Ohio. 



Unio RADIATUS, Lam., fig. 20. Pouch-shape ; dorsal line long, rounded at each end and furnished with a 

 small spur-like process on the inside of each valve at the angle;* side margins slightly curved ; basal 

 margin nearly semicircular ; clear white. Has four small spur-like processes, one at each angle of the 

 valve in the interior. Part of the second valve is annexed. Fig. 20 a represents a single sack of the 

 natural size, being nearly half an inch long and one twentieth thick, and contained about 1200 eggs 

 with the young well developed. Mohawk, New York. 



Unio perplextjs, Lea, fig. 21. Subrotund ; dorsal line very long, slightly outwardly curved in the middle, 

 obtuse at the ends; side margins rounded; basal margin semicircular; clear white. Has no hooks. 

 Length 0.224 mm. Breadth 0.24 mm. Columbus, Ohio. 



Unio undulatus, Bar., fig. 22. Subtriangular ; dorsal line very long ; obtuse at the ends ; side margins more 

 curved above than below ; basal margin subangular ; dark brown. f Has no hooks. Columbus, Ohio. 



Unio pressus, Lea, fig. 23. Subtriangular ; dorsal line slightly curved, much rounded at the ends ; side 

 margins round above and compressed below to a point; basal margin angular, furnished with a hook; 

 light brown. A hook is placed on the inside of the angle of each valve at the base. Fig. 23 a shows 

 the form of the hook which is greatly magnified. It has an arrow-head-like point and is furnished with 

 two rows of minute tubercles passing from the point to the base. Fox Biver, Illinois. 



Unio l^vissimus, Lea, fig. 24. Wedge-shape; dorsal line very short and straight; side margins above 

 slightly incurved towards the base, much curved outward ; basal margin mnch curved, wider than 

 dorsal, forming acute angles, with a hook-like process on each angle ; clear white. Hook-like pro- 

 cesses very small, — four, — one on each angle of the basal margin. Both valves inflated, being opened 

 at both sides. Fig. 24 a is a side view of the valves lying open. It was difficult to represent this per- 

 fectly. 24 b is an oblique view, showing the interior. Iowa. Length 0.16. Breadth at dorsal line 

 0.048. Breadth at base 0.104. Diameter of the two valves 0.096 mm. 



Unio alatus, Say, fig. 25. Wedge-shape ; dorsal line short and straight; side margins above slightly 

 curved outward, towards the base curved inward ; at the base much curved outward ; basal margin 

 slightly curved, wider than dorsal, forming acute angles with a minute hook-like process on each an- 

 gle ; clear white. Hook-like processes very small — four — one on each angle of the basal margin. 

 Both valves inflated, being opened at both sides. Fig. 25 a is an oblique view, showing the interior. 

 Fig. 25 b is an end view, showing two small hooks at the base, and the adductor muscle above. Cin- 

 cinnati, Ohio. 



Margaritana RUGOSA, Lea, fig. 26. Sub-triangular; dorsal line very long and straight ; side margins in- 

 flated above, compressed and flattened below, forming an obtuse angle at base ; basal margin obtusely 

 angular, furnished with a hook ; light brown. A hook is placed on the inside of the angle of each valve 

 at the base. Fig 26 a is a view into the interior of the partly open valve, and exhibits the two hooks 

 with their points nearly in contact. Fig. 26 b is the hook very much magnified, broad at the base, and 

 terminated by an arrow-headed point. There are a number of rows of granules converging towards the 

 point. Length 0.4. Breadth 0.368. Hook 0.112. Granules 0.0045 mm. Mohawk, New York. 



* I could not observe this on all the specimens. They may exist only on the mature embryonic shell. Part 

 of the other valve is represented as showing the processes distinctly. 



f Some in the same branchial uterus were white, but probably not so much advanced. 



