FRESH-WATER CRUSTACEA OF THE UNITED STATES. 659 



ASELLOPSis Harger. 



American Jourual of Science, 3d series, vol. vii, p. 601, 1874. 



The geuas Asellopsis, which was proposed for the reception of Asellus 

 tenax Smith, differs from the genus Asellus in .the absence of mandibular 

 palpi. The presence of these palpi has been heretofore regarded as 

 characteristic of the family to which both the genera undoubtedly 

 belong. 



ASELLOPSIS TENAX Harger. (Plate I, fig. 3.) 



Asellus tenax Smith, American .Journal of Science, 3d series, vol. ii, p. 453, 1871 ; 

 and Preliminary Report on Dredging in Lake Superior, p. 1023, 1871. 



Anterior margin of the head broad, excavated for the bases of the 

 antennulse; external angles rounded; margin expanded with a large, 

 rounded sinus on a line with the eyes ; behind this the margin expands 

 into a rounded lobe. The posterior margin of the head is broad and 

 rounded behind, adapted to the first thoracic segment. Eyes of more 

 than twenty facets, considerably within the margin of the head, oval or 

 somewhat reniform. Antennaj about half as long as the body, sepa- 

 rated from each other at the base by about half their diameter ; first 

 three segments shorter than broad, of about equal length, successively 

 decreasing in diameter; fourth segment as long as the first three, cylin- 

 drical ; fifth or last peduncular segment as long as the third and fourth 

 together, slender, slightly clavate ; flagellum of about thirty segments. 

 Immediately exterior to the base of the antenna is a prominent tubercle, 

 tipped with a few short bristles. Autennulje with the basal segments 

 large and swollen, about equal in diameter to the fourth segment of the 

 antennte; second segment slender, slightly clavate, about attaining the 

 end of the third segment of the antennae ; third or last peduncular 

 segment small and slender, less than half the length of the second 

 and similar to the segments of the flagellum, which are usually five in 

 number, the second being longest. 



First thoracic segment concave forward, as is also the second in a less- 

 degree ; third about straight posteriorly ; last four slightly curved in 

 the opposite direction. Pleon narrowed posteriorly, obtusely rounded 

 at the end. The margins of all the segments, as well as the pleon and 

 the head, are ciliate ; the cilia being more abundant along the external 

 margins of the segments. 



Left mandible with two dentigerous lamellae ; molar process truncated 

 nearly at right angles; right mandible with a single dentigerous lamella ;. 

 the molar process obliquely truncated. Palpus of the maxilliped with the 

 first segment short, nearly cylindrical ; second segment suborbicular,with 

 about five cilia along the external margin and twenty along the internaL 

 Cilia much fewer than in A. communis ; third segment truncate-oval in 

 outline, somewhat broader than long, ciliate; fourth segment cylindrical, 

 or somewhat clavate, less than half the diameter of the third ; fifth or 

 last segment about half as long as the penultimate. 



