76 MEMOIRS OF THE CARNEGIE MUSEUM. 



and even Simpson did not recognize it clearly. In fact, two of the specimens of 

 which he published the measurements (the first two) fall under the var. coccineum. 

 The synonymy given by Simpson is correct, as far as it goes. But there is no doubt 

 that U. catillus Conrad (1836), which is made by Simpson a synonym of U. coc- 

 cineus should be united with U. solidus Lea. Its diameter is fifty-one percent of 

 the length (according to Conrad's figure, PL 13, fig. 2), and its greater obesity is 

 precisely the character which distinguishes it from U. coccineus. U. solidus (ac- 

 cording to Lea's figure) has a diameter of sixty percent of the length, while U. 

 coccineus Conrad {ibid., PI. 13, fig. 1) has the diameter thirty-seven percent of 

 the length. 



That U. catillus more nearly approaches U. solidus, was already recognized 

 by Utterback, who also clearly saw that these forms in Missouri pass into each 

 other and into obliquum. Utterback tried to express this in a very peculiar way 

 by naming one of the intergrades P. catillus, and another P. obliquum catillus, but 

 this can hardly find approval. 



The Carnegie Museum possesses, from the Hartman collection, a specimen 

 (Cat. No. 61.1440) from Cincinnati, labeled U. solidus, "type" (meaning typical), 

 and "Lea datum." This, consequently, is an authentic specimen from Lea, from 

 one of the type localities. It has a diameter of sixty-four percent, and thus cor- 

 responds closely with Lea's original figure, and with my conception of this form. 



That U. solidus of Call (1900, PL 59, upp. fig.) is a typical P. obliquum, has 

 been mentioned above. What Scammon (1906, PL 85) figures as a young Quad- 

 nda solida, is not this, but a rather good P. obliquum rubrum. 



Unio fulgidus Lea (Obs. IV, 1848, PL 4, fig. 10), supposedly from Alexandria, 

 Louisiana, is made by Simpson a synonym of Q. solida. It is founded upon a very 

 young specimen, and I hardly think that it is possible to make out what it reaUy is. 

 However, Walker has communicated under the name of fidgidus a specimen from 

 Iowa City, which undoubtedly' belongs to solidus (= catillus). It has a rather 

 shining epidermis, but otherwise closely resembles specimens labeled solidus by 

 Walker. Such specimens seem to prevail west of the Mississippi, but I have them 

 also from the Tennessee-drainage. 



In Pennsylvania, P. obliquum catillus may be recognized by the absence of the 

 radial furrow, the sutriangular outline, and considerable obesity. By the reduc- 

 tion of the latter, i.e., when the sheU becomes more compressed, it passes very 

 gradually into the var. coccineum, and it is possible to separate the transitional 

 forms only by exact measurements, and an artificial dividing line. I have drawn 

 this line at the diameter of fifty percent of the length, but I again must emphasize 



