176 Illinois State Laboratory of Natural History. 



abdominal spine mentioned by Marsh, but since the speci- 

 mens from the two localities differ in other respects it is not 

 unlikely that this difference also may exist. 



Found by Professor Marsh in collections from small lakes 

 and ponds in Mississippi, and by the writer in material col- 

 lected from a number of Florida lakes in March, 1896, by 

 Mr. Adolph Hempel. Professor Marsh states that in collec- 

 tions made in Mississippi in January and February, 1893, 

 D. mississij)2'iensis was the only Diaptomus found ; but in the 

 Florida collections this species occurred in connection with 

 I), alhuquerquensis Herrick, the two being about equal in 

 numbers. In general appearance these two species are very 

 similar, but they are widely different in the details of their 

 structure. 



Diaptomus tyrrelli Poppe. 



Diaptomus tyrrelli, Poppe, '88, p. 159. 



Diaptomus tyrrelli, de Guerne et Richard, '89b, p. 39, PI. I., Fig. 



17,18; PI. IV., Fig. 26. 

 Diaptomus fresnanus, Lilljeborg, i7i litt* 

 Diaptomus tyrrelli, Herriclv and Turner, '95, p. 76, PI. X., Fig. 9. 



"Of medium size. Cephalothorax widest at the middle 

 and at the lateral lobes of the last segment. Last two tho- 

 racic segments confluent above, and the last, seen from above, 

 produced obliquely into large lateral processes, almost ovate, 

 acuminate posteriorly, with a rather large mucro. First 

 abdominal segment almost as long as the rest of the abdomen, 

 somewhat dilated anteriorly, and provided with long mucro- 

 nate lateral processes. Second and third abdominal segments 

 rather short, with a vestige of a transverse suture anteriorly. 

 Furcal rami short, sparsely hairy, and almost one and a half 

 times as long as broad. 



"First pair of antennae in the female, reflexed, reach 

 almost to, sometimes to the end of, the furca ; composed of 

 twenty-five segments. Antepenultimate article of the pre- 

 hensile antenna of the male wholly unarmed, or minutely 

 and scarcely perceptibly armed. 



*The description given herewith is that of D. fresnanus Lilljeborg, sent by him 

 to de Guerne and Richard, and published by them under the name of D. tyrreUi. 



