282 ORTMANN—CORRELATION OF SHAPE AND 
into this problem, which may necessitate some radical changes in our 
Naiad-nomenclature. 
This much is certain, that F. pilaris, the representative form in 
the upper Tennessee of F. subrotunda from the Ohio, behaves 
exactly as F. subrotunda, but the headwaters-forms of the two, in 
the upper Tennessee and the upper Ohio drainages, although very 
similar, have no direct genetic connection, and undoubtedly have 
developed independently of each other. 
Group oF FUSCONAIA FLAVA IN THE UPPER OHIO-DRAINAGE. 
I classify the forms belonging here as follows. 
I. FUSCONAIA FLAVA (Rafinesque).—Dia. less than 55 per cent. 
Obliquaria flava Rafinesque, ’20 (trib. of Kentucky, Salt, Green R.). 
—Quadrula flava Vanatta, Pr. Ac. Philad., ’15, p. 557. 
Dias (Conrad, 37): 43" per cent.; (Vanatta) 30 per cent: 
Unio rubiginosus Lea, ’29 (Ohio).—Quadrula rubiginosa Simpson, 
BI Woz: 
Dia. (luea’)/: 44 per cent.; (Simpson): 41, 34,53 per cent. 
2. FUSCONAIA FLAVA TRIGONA’ (Lea).—Dia. 55 per cent. and 
over. 
Unio trigonus Lea, ’31 (Ohio R., Cincinnati and Louisville) — 
Synonym to Quadrula undata (Barnes), according to Simpson, 
‘TA, p. Si. 
Dias (eas O2)peracent: 
Probably also U. undatus Barnes, ’23, belongs here. This form 
has been discussed by Walker (Nautil., 24, ’10, p. 24), who says 
that it is the same as trigonus. He gives the diameter of 68 per 
cent. Simpson (1. c.), who follows him, gives it as 60, 61, and 76 
per cent. 
Walker and Simpson probably are right in regarding trigonus 
and undatus as synonyms, as far as it concerns obesity. However, 
I think that they differ in the development of the beaks, undatus 
having higher beaks, so that the outline is more triangular, while it 
is subtrapezoidal in trigonus. Of typical undatus I have no good 
material, and this form is not found in the upper Ohio. What I 
have from this region (vicinity of Pittsburgh and upwards) repre- 
