628 



MEMOIRS OF THE CARNEGIE MUSEUM. 



Simpson also gives Peru, but I do not know on what authority, and strongly doubt 

 this record. 



Von Ihering (1890) pointed out the differences from A. riograndensis, to 

 which this species is closely allied. The original specimens of A. forbesiana were 

 injured at the posterior end, and it was believed that in iminjured bnes this end 

 might be more strongly produced. However, Lea's figure shows in the growth- 

 lines that the shape of the shell, before it was injured, was similarly truncated, and 

 my series, which contains a majority of intact shells, demonstrates that the normal 

 shape of this species also exhibits this truncation, i.e., a steeply descending pos- 

 terior margin. This is the chief character of this species, and in consequence, the 

 dimensions are different from those of A. riograndensis. 



This difference is seen first of all in the height, which varies from about 60 to 

 69 pr. ct., falling under 60 j)r. ct. only in very young specimens, while in riograndensis 

 it is from 54 to 63 pr. ct. The diameter of the two species is about the same, but 

 slightl}' higher on the average in forbesiana (35 to 48 pr. ct. and in young ones 

 as low as 33 pr. ct. against 34 to 40 in xingrandensis). The beaks of forbesinna 

 are farther removed from the anterior end: 32 to 38 pr. ct., against 29 to 33 

 pr. ct. in riograndensis. Von Ihering gives the umbonal index as 48 pr. ct., but 

 this is due to his different method of measuring; measured in mj' specimens ac- 

 cording to his method, it would be from 42 to 56 pr. ct., while it is 34 to 43 pr. ct. 

 in riograndensis. The index above 50 pr. ct. is found only in my youngest speci- 

 mens, where the ligamental sinus is very anterior, thus making the hinge-line very 

 short. 



Finally, in forbesiana, the hinge-line is practically straight, as mentioned b}' 

 Von Ihering, and represented in Lea's figure. This holds good in all of my speci- 

 mens. 



In other respects this species agrees with riograndensis, but it should be re- 

 marked that the radial scalariform stripes are generally' absent or very poorly 

 developed; only in one or two cases a few of them are distinctly seen on the anterior 



part of the shell. 



Measurements (Specimens from Uruguayana). 



