CE ACP aN. Vill: 
THE LOWER SILURIAN OSTRACODA 
OH MINNHSOTA. 
BY E. O. ULRICH. 
This order of Crustacea comprises small, generally minute, animals having the 
entire body enclosed in a shell or carapace consisting of two more or less nearly 
equal calcareous or corneous valves, united along the back by a membrane, and 
capable of being opened at their ventral margins. The valves are closed by a sub- 
central adductor muscle, the attachment of which is marked on their inner sides by 
a tubercle, a pit or a number of small spots. The body is not segmented but has 
seven pairs of appendages, of which the first two are antenne, while they, like the 
others, are also adapted for creeping and swimming. These appendages, together 
with the caudal extremity of the short abdomen, are protruded along the ventral 
margin of the carapace when the valves are opened. 
Behind the first two pairs of appendages (antennules and antenne), is a pair of 
mandibles, followed by a pair of maxillee, while the third and fourth pairs may be 
either legs or jaws. Finally the last two pairs are leg-like and generally stronger 
than the preceding pairs. The extremity of the abdomen may be bifurcated or con- 
sist of a single spinous plate. 
As a rule the eyes are well developed, with commonly a small median and two 
larger lateral ones present. The position of the latter is often indicated on the 
exterior of the valves by a small “eye tubercle.” A distinct heart may be present 
or absent, but the alimentary and generative organs are well developed. 
The Ostracoda, or “ water-fleas” as they are often called, are represented by very 
numerous forms both in fresh water and in the sea. Of the families only the Cypride 
are chiefly fresh-water forms, while most of the other families are restricted to 
marine or brackish waters. Taken as a whole they are to be considered as shallow 
water inhabitants, and of social habits, being found in great numbers swimming near 
the surface of the water or creeping over the bottom. Remains of Ostracoda abound 
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