264 llie Amerlcnn Geologist. November, isna 
greatest trouble seems alwa3-s to have Ijeea experienced in draw- 
ing the lines between the various genera constituting the large 
family Lcpcnlitlula, some authors claiming the existence of a 
complete chain of links connecting the simplest Primititi-aye^ 
even Leptrdltla — with the most complex types of Bei/richia and 
Strepula. Now, wdiile this is probabh' true, it seems to me that 
but too often these chains contain closely approximated links that 
in reality illustrate mere resemblances rather than genetic relation- 
ship. 
It is the determination of the genealogical affinities that is the 
prime necessity in building up a permanent classification. With 
nothing ])ut the shells to guide us, can we hope to reach satisfac- 
tory conclusions respecting the paleozoic., especially the lower 
paleozoic, Ostracoda? We can try. 
In the accompanying plate several forms are illustrated of 
which it is reall}' difficult to decide whether the}' should be called 
species or varieties. One thing, how.ever, seems to be certain, 
and that is that, within reasonable limitations, each form is con- 
stant and, therefore, distinguishable from the others. That they 
represent recognizable stages in the developmental history of the 
Lepcrditiido' is all that I claim, it being quite immaterial to me 
what rank in classification the proposed names may ultimately 
hold. 
Leperditia tumida, n. sp. 
Plate ix, Figs. 1 to 3. 
Length of a large right valve, 2.6 mm.; hight, 1.82 mm.; thickness, 
0.75 mm. 
Valves ovate, rather short, the posterior end much the widest, 
tumid, the convexity of the surface, except for a slight flattening 
and lengthening of the dorsal and anterior slopes, nearly 
uniform. Surface obscurely punctate, otherwise smooth, there 
being no external signs of either the eye-tubercle or muscle spot. 
This species compares with L. nana Jones, about as closeh' as 
with any other Lower Silurian form described, but being without 
an eye-tubercle and of more rounded shape, it is readih' distin- 
guished. The Carboniferous species L. ohni Miinster, L. car- 
honaria Hall, and others of that type, agree very closely in nearly 
every respect. All these species differ from typical Leperditia 
in the method of closing the valves, the latter having them merely 
to overlap, while in the species under consideration the ventral 
