44 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 39. 



priority over "no turn" as a general term, the writer has used the 

 latter in the more restricted sense, and has named the posterior sec- 

 ondary sclerite of the tergum, when present, the "postnotum." 



The lateral margins of each wing-bearing notum are produced into 

 the anterior and posterior notal wing processes (figs. 1,2, and 4, ANP 

 and PNP) for the articulation of the wings. The ventral surface very 

 commonly presents three ridges — an anterior notal i^idge (fig. 3, ANR), 

 a posterior notal ridge (PNR), and a median y -shaped notal ridge 

 (VNR) — the "entodorsum," having its apex forward. These three 

 ridges form three transverse lines (fig. 2, anr, vnr, and pnr) on the 

 surface of the notum. The first is slightly submarginal on account 

 of the reflexed anterior edge of the notum, while the third is nearly 

 always some distance in front of the posterior edge of the notum, 



ANP— 



PNP 



ANR 



AxC 



par 



PNR VNR, 

 Fig. 3.— Ventral view of mesotergum of Bl\- 



TELLA GERMANICA: ANR, ANTERIOR NOTAL 



ridge; pnr, posterior notal ridge; VNR, 



MEDIAN V-SHAPED NOTAL RIDGE, THE "ENTO- 



dorsum;" other lettering as in fig. 2. 



Fig. 2.— Mesotergum of Blatella germanica 

 (cockroach), dorsal view, illustrating a 

 tergum consisting of a notal plate alone: 

 a, chitinous fold reflected upon posterior 

 EDGE of protergum; ANP, anterior notal 

 WING process; anr, line formed by anterior 



VENTRAL NOTAL RIDGE; AxC, AXILLARY CORD; 

 PNP, POSTERIOR NOTAL WING PROCESS; pm, UNE 

 FORMED BY POSTERIOR VENTRAL NOTAL RIDGE; 

 Rd, POSTERIOR REDUPLICATION OF THE NOTUM; 



vnr, LINE formed by median ventral V-shaped 



NOTAL RIDGE. 



which forms a conspicuous posterior reduplication of varying width 

 (figs. 1, 2, and 3, Rd) overlapping the part behind. 



Finally, the notum is commonly more or less divided into three 

 regions by topographical differentiation or by transverse lines or 

 sutures, independent of those formed by the ventral ridges. The 

 first subdivision is the prescutum, the second the scutum, and the third 

 the scutellum. These are best marked in the higher forms, as illus- 

 trated by the mesotergum of a cranefly (fig. 4, Psc, Set, Scl), and are 

 clearly not homologous in all the orders, because they do not always 

 bear the same relation to the more fixed characters of the notum. 

 In the Hymenoptera the notum is actually cut into two separate 

 pieces by a suture crossing it in front of the apex of the V-shaped 

 ridge (pi. 10, fig. 46, Tc). In the lower orders the differentiations of 

 the notum are largely topographical. In the cockroach (fig. 2) there 

 are no divisions corresponding with those of the cranefly (fig. 4), 



