92 - THE AMERICAN MIDLAND NATURALIST 
Peck in his “Study of the Lamellibranch Gill’. (Quart. 
Jour. Mic. Sci.) (5) investigated the histological structure of 
the gills with the object of ascertaining the morphological 
equivalent of the gill tentacles of other more primitive Pele- 
cypoda. As his work was entirely on marine species of course 
his work’ is without mention of the furrow which is the sub- 
ject of this paper. The author however has found as would 
be expected, many points of similarity between the histolog- 
ical structure of the gills of the Najades and that of the forms 
investigated by Peck. Points of similarity will be mentioned 
later. 
The material used in the investigation was entirely from 
the Mississippi drainage system and consisted of the follow- 
ing species (nomenclature after Ortmann): Lampsilis sili- 
quoidea (Barnes), Anodonta corpulenta (Cooper), Amblema 
peruviana (Lam.), Elliptica dilitatus (Raf.), Lasmigona com- 
planata (Barnes), Obliquaria reflexa (Raf.), Fusconaja un- 
data (Barnes), Proptera alata (Say), Lampsilis fallaciosa 
(Smith). 
IIL—METHOD OF INVESTIGATION, 
About fifty different specimens of various ages, and ones 
representing the different species named in the introduction 
furnished the material for this study. Such a selection was 
necessary because Ortmann has shown that in different spe- . 
cies there is considerable variation in the structure of the gill 
especially during the marsupial period (4), and it was desired 
that the study of the histological structure should be as general 
as possible with the material at hand. 
In every case live mussels were used and when opened the 
gills were cut out, placed in the different fixatives, and label- 
led according to name, sex, left or right, inner or outer, gill. 
In a few specimens the sex as judged from the apearance of 
the shell ‘was on microscopical examination found to be erro- 
neous and the data changed accordingly. The septa in the 
female separating the water-tubées in the marsupial gill are 
much more crowded than in the male, aid in the final decision 
as to sex this point was given preferente over shapes of the 
