74 



Guide to the Invertebrata. 



GALLERY we now have a very complete knowledge of Triarthrus Becki, 

 East Side. ^^"^™ ^^^ Ordovician formation of Home, New York, and a more 

 "Wall-cases ^^ ^^^^ complete knowledge of Asaphus, Calymene, Ceraurus, 

 13 & 14, Trinucleus, etc. The fii st pair of antennae have a simple, many- 

 jointed lash (or flagellum) attached to the under-side of the head 

 on each side of the upper lip {hypostome). The second pair of 

 antennae have two branches, and the basal joint is triangular 

 in shape and bears a masticating edge. The third pair of limbs 

 are also biramous and correspond with the second pair, and may 

 be compared with the mandibles of other Crustacea, being mouth- 

 organs at their base. Two pairs of gnathites, or maxillce, follow ; 



80-82. 



(1) 



(2) 



Fig. 122. — Triarthrus Becki, Green. Utica Slate (Ordovician) : Eome, 

 New York. 



(a) Original specimen figured by Walcott. 



(1) Upper and (2) under side (restored by Dr. C. E. Beecber). 



they resemble the second and third pairs, but are somewhat larger ; 

 they all are used as mouth-organs, assisting in mastication. Behind 

 them is placed a small oval metastoma, or lower lip. Fourteen pairs 

 of biramous legs follow, each limb having a seven-jointed walking- 

 leg (the endopodite) and a swimming-branch fringed with hairs 

 (the exopodite) attached to its base. The anterior legs are the 

 longest, the others very gradually becoming shorter towards th6 

 pygidium. The pygidium carries six pairs of appendages, each 

 of which is also biramous, but the joints are transversely expanded 

 so as to form broad lamellae which probably assist in respiration, 



