10 Annals of thk Carnegie Museuim. 



The following key, taken, with a few changes, from works by Hagen, 

 Faxon, and Hay, will aid in identifying Allegheny county specimens. 



I. Third segment of only the third pair of legs of the male hooked. 



I. First pair of abdominal appendages of male ending in two thick recurved 

 teeth ; rostrum in both sexes without lateral teeth. 

 ■;. Areola wide, form depressed ; brook species. 



(1) Rostrum wide, sides subparallel. C. baitoiiii. 



(2) Rostrum narrower, sides tapering to the ape.\. 



C. bartoiiii, var. robintus. 

 h. .'\reola narrow or obliterated ; form compressed ; burrowing species. 



( 1 ) Areola obliterated, color in life olive, greenish, or yellowish. 



C. diogenes. 



(2) Areola narrow, color in life blue, or showing traces of blue. 



C. d It hi us. 



II. P'irst pair of abdominal appendages of male ending in two elongated, nearly 

 straight, acute tips ; rostrum in both sexes with lateral teeth. 



a. Sides of rostrum straight ; al)dominal appendages shorter. 



C. propinqiius. 

 Ik Sides of rostrum concave, abdominal appendages longer, more tapering. 



C. 7- US ti cits. 



The technical terms em])loyed in this key may be briefly defined. 

 For further details the student should consult any of the larger works 

 on invertebrate anatomy. Crayfish are decapod (ten-footed) crusta- 

 ceans. The first three pairs of legs are armed with pincers, which are 

 most strongly developed on the first pair of legs, where they are known 

 as chelce. In all species the males may have the third .segments of one 

 or two pairs of legs hooked, that is armed with a prominent tubercle. 

 The abdoiiiiiial appt'/iJagcs correspond in position to the legs, and are 

 attached, one pair to each segment, to the abdomen. In the males 

 the first two pairs of these are modified, and the first pair are turned 

 forward and pressed closely between the bases of the legs. Their 

 various modifications offer the most valuable of all specific characters. 

 The head and thorax are united to form the cephalothorax. This is 

 covered by a firm chitinous shield, the carapace. The carapace is 

 prolonged in front between the eyes to form the rostrum. The apex 

 of the rostnun is more or less triangular in form, one angle of which 

 forms the extreme a])ex of the rostrum. The other two angles may 

 be rounded into the sides of the rostrum or they may be armed with 

 spines, the lateral teeth. Near the middle of the carapace is a trans- 

 verse groove, the cervical groove. Back of the cervical groove are two 

 transverse grooves, and the space between them is the areola. When 



