502 



MEMOIRS OF THE CARNEGIE MUSEUM. 



while the other, somewhat more oblique, is more frequent ; Init both pass insensibly 

 into each other. According to my material younger specimens are more apt to 

 exhibit the more regularly ovate outline. 



This species is not veiy closely allied to those mentioned on the jireceding 

 pages, but represents a somewhat different type, and inclines towards the elliplicus- 

 group by its often slightly oblique shell. It is much smaller than imitator, vicarius, 

 and decipiens, and also does not attain the size of mnillimus. The largest specimen 

 before Von Ihering was 57 nun. long, while my largest falls even short of this (45 

 mm. from Rio Tiete, 51 mm. from Mogy Mirim). The subtrapezoidal shape is only 

 distinct in very young individuals; in larger specimens it becomes subelli])tical or 

 subovate, generally a little higher posteriorly and slightly oblique. The ground- 

 color of the epidermis is greenish-olive, and never distinctly brownish, although old 

 specimens may become blackish green. The beak-sculpture consists of fine, sharp, 

 and short radial bars, the number of which is sixteen to eighteen, the median pair 

 having a tendency to unite at the lower ends. 



The characters of the inside of the shell have been well described by Von 

 Ihering. The left valve has generally only one pseudocardinal, but sometimes 

 there is a trace of a second posterior one. It should be noted that, as described 

 by Von Ihering, the posterior retractor-scar is separated from, and stands above 

 the posterior adductor-scar, but not always, as in some cases it is connected with 

 it, and this may be different, even in the right and left valve of the same individual. 



Measurements. 



No. 



Sex. 



Length. 



Height. 



Diameter. 



Beaks. 



Von Ihering .... 

 Mogy das Cruzta 



18 



10 



23 



7 



14 



Mogy Mirim 



9 



14 



V.i 



12 



a 



5 



48 to 57mm. 



15.5 mm. 



27 



38 



38 



41 



45 



35 

 39 

 40.5 " 



42 



4S.5 " 

 51 



59 to 65 pr. ct. of L. 

 8.5 mm. =55 " 



16 



22 



24 



27.5 



27 



22 



23.5 



24 



28 



29 



32 



= 59 

 = 58 

 = 63 

 = 67 

 = 60 



= 63 

 = 00 

 = ,'-.8 

 = 67 

 = 60 

 = 63 



31 to 35 pr. ct. of L. 



4 mm. =26 " 



.S " =30 



11.5 " =30 



14 " =37 



14 ■' =34 

 16 ■■ =36 



14.5 " =41 



13 '• =33 



15 " =37 

 IS " =43 

 16.5 " =34 

 18 " =35 



22 to 26 pr. ct. of L. 



at 



4 







7.5 

 10 

 10 

 11 



10 



9.5 

 11.5 

 11.5 

 13 

 13 



mm. =26 

 " =22 

 •■ =20 

 " =20 

 •' =24 

 •' =24 



" =29 

 = 24 

 = 28 

 = 27 

 = 27 

 = 26 



My two sets from the Rio Tiete and from a creek tributary to Rio Mogy 

 Guassu, differ slightly from each other. In the former the size is a little smaller 

 and the young shells are rather more elongated, thus rendering the average height 

 less. The diameter is also not so gr(>at in the shells from the Tiete. The measure- 



