CHAPTER Vir. 



THE LOWER SILURIAN OSTRACODA 



OF MINNESOTA. 



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 antennae, 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 antennae), is a pair of 

 mandibles, followed by a pair of maxillae, 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 singlie 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 Cypridce 

 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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