286 



UNIVERSITY OF COLORADO STUDIES 



92 (93) Beak curved sharply forward and upward at the tip. Teeth along 

 entire posterior margin of valves. Length, o. 5 mm. 



Pleuroxus procunalus Birge 



Tolland, Twin Lakes; 8,150-9,200 ft. [Northern United States.! 



93 (92) Beak not curved upward. Small teeth at lower posterior margin of 

 valves only 94 



94 (95) Post-abdomen rounded at posterior margin; 9-12 spines. Length, 

 0,6 mm Pleuroxus adunctus (Jurine) 



Boulder; s.200 (t. [Colorado and California. Northern Europe to Algiers and Syria.] 



95 (94) Post-abdomen with prominent angle at posterior margin. Length, 

 0.6 mm Pleuroxus denticulatus Birge 



Boulder; 5,200 ft. [Common in all parts of United States.! 



Fig. 55. — Pleuroxus adunctus. Fig. 56. — Pleuroxus denticulatus. 



Fig. 54. — Pleuroxus p 

 curvalus. 



96 (91) Beak short and blunt. Shell with markings more or less definitely 

 hexagonal Genus Alonella 97 



This describes only part of the genus. Other species not included in our faima would not fall under 96. 

 The genus is poorly defined. 



97 (98) Valves with distinct, fine longitudinal striations as well as hexagonal 

 reticulations. Lower posterior margin of valves with rounded teeth. Post- 

 abdomen of this species commonly described as angled at apex but in most 

 of my specimens it was rounded as figured. Length, o. 5 mm. 



Alonella excisa (Fischer) 



Tolland; 8,100-8,475 ft. [Common in United States, Greenland and all of Europe.) 



98 (97) Valves with hexagonal markings only. Teeth at lower posterior margin 



Fig. 57. — Alonella excisa. a, female; b, markings of valve. 



