298 MEMOIRS OF THE CARNEGIE MUSEUM. 



Similar to L. ovata ventricosa, but distinguished by the development of the 

 posterior ridge, which is ver.y sharp and very distinct toward the beaks. In 

 consequence of this the posterior slope is flattened, and in most cases even concave, 

 chiefly towards the beaks. Further towards the anterior end of the shell the two 

 valves are not uniformly convex, but peculiarly compressed, rendering the hori- 

 zontal cross-section of this part wedge-shaped. 



In addition there is a tendency toward the suppression of the rays. Rays are 

 indeed sometimes present, but in most cases the epidermis is uniformly greenish 

 3^ellow, dark upon the posterior slope and without rays. Old males become drawn 

 out at the posterior end, almost rostrate. 



L. . H. D. 



Size: (Males) 1. Aladdin, Cat. No. 61.3426 136 mm. 92 mm. 57 mm. 



2. Walnut Bend, Cat. No. 61.3423 103 



3. Godfrey, Cat. No. 61.4009 86 



(Females) 4. Industry, Cat. No. 61.3425 126 



5. Cambridge Springs, Cat. No. 61.4010 (gravid) . 100 



6. Cochranton, Cat. No. 61.3420 83 



Soft parts (See Ortmann, 1912, p. 350, fig. 26). Glochidia (See Lea, Obs. VI, 

 1858, PI. 5, fig. 15). I have found them to measure: 0.24 X 0.28 mm. 



Breeding season: My records for gravid females cover the period from August 

 4 to October 24, and from May 23 to May 25. The spring records are scanty, but 

 the species is surely hradytictic, and there is an interim at least in July. 



Remarks: There are numerous intergrades between L. ovata and L. ventricosa. 

 In the larger rivers, the two forms are practically always associated, and the 

 transition from a very sharp to an almost entirely effaced posterior ridge with 

 the corresponding intermediate condition of the posterior slope, is frequently found. 

 Likewise the other characters given above pass into each other. In color there is 

 also great variability, and, although the true ovata has generally no rays, rays are 

 sometimes very well-developed; on the other hand in true ventricosa the rays may 

 be lacking. I have quite a number of specimens, which are so completely inter- 

 mediate, that I am unable to assign them to either form. 



Going up beyond a certain point in our rivers, and into the smaller tributaries, 

 L. ovata disappears, and its place is entirely taken by L. ovata ventricosa, and there 

 are many creeks, where only the form ventricosa exists with no trace of ovata, and 

 no indications of an inclination toward it. 



Exactly the same conditions prevail in the upper Tennessee-drainage in 

 eastern Tennessee and southwestern Virginia, and a form corresponding to L. 

 ovata ventricosa develops out of the true L. ovata in the headwaters. However, the 



