CR USTA CEA—MALA COSTRA CA . 



381 



front of mid-length, with a single carina ornamented by oblique 

 striae. (Type of genus.) 

 Chemung of Pennsylvania. 



XI. Tropidocaris Beecher. 



Bivalve carapace with truncate posterior margins divided by 



median lanceolate plate and an elongate rostral plate in cephalic 



region; eye node well defined; other nodes of cephalon obscure; 



rostrum narrow and ridged; surface of valves with one or more 



Fig. 1686. Tropidocaris bicarinata^ioXdi^d, Fig. 1687. Tropidocaris alternata, 

 carapace with abdomen, telson and cercopods entire left valve, X |- (After Beech- 

 exposed, X 7- (After Beecher.) er. ) 



strong, longitudinal keels ; abdomen with two exposed segments, 

 which are subcylindrical and without small spines. Upper Devonic- 

 !Mississippic. 



21. T. bicarinata Beecher. (Figs. 1685, 1686.) Devonic. 

 With two strong, lateral carinse, and a shorter intercalated one 



in cephalic region. (Type of genus.) 

 Chemung of Pennsylvania. 



22. T. alternata Beecher. (Fig. 1687.) Mississippic. 

 Valves elongate with seven alternating longitudinal carinae, and 



two spiniform prolongations on the posterior margin, the continua- 

 tions of the fifth and sixth carina; stronger carinae with pits on 

 summit; several minor intercalated ones in cephalic region. 

 Waverly of Pennsylvania. 



