NO. 2 THORACIC MECHANISM OF A GRASSHOPPER SNODGRASS 49 



contains the external impressions of the sternal apophyses (fig. 30 A, 

 sa, sa). The suture, therefore, is the f ureal suture of the mesoster- 

 num, and the two postero-lateral, quadrate mesosternal lobes (Sh, 

 Sin) lying- laterad of the median projection of the metasternum must 

 belong to the sternellar region of the mesosternum. A median pit 

 (spti) opening just behind the f ureal suture (k) marks the site of the 

 internal spina (fig. 31, 2Spn), which normally is intersegmental be- 

 tween the mesosternum and the metasternum, but which is here fused 

 with the mesosternal f ureal ridge {k). There can be no doubt, there- 

 fore, that a part of the mesosternum normally intervening between 

 the furcal ridge and the spinasternum, which is the median area of the 

 mesosternellum (fig. 30 B, SI), has been obliterated in Dissosfeira, and 

 that the spinasternum itself has been reduced to little more than the 

 base of the spina. 



The mesosternum of Dissosteira is thus to be analyzed into the same 

 structural elements that are preserved in a less modified form in the 

 prostenium. The sternellar region of the sternum (fig. 30 B, SI, SI) 

 has been cut into a pair of lateral lobes (A, Sh, SI2) by the suppres- 

 sion of its median area, and the following spinasternum (B, Ss) has 

 been reduced to the base of the spina (fig. 31, 2Spn), which is united 

 with the furcal ridge {k). 



The endoskeletal features of the mesosternum consist principally of 

 the strong furcal ridge (fig. 31, k, k) and the two sternal apophyses 

 {SA, SA). The latter are broad, tapering plates arising from thick 

 bases and extending laterally beneath the pleural apophyses, to which 

 they are attached by short muscle fibers (figs. 34, 35, 86). Each has 

 a triangular basal lobe directed forward. Anteriorly the mesosternum 

 is marked by the sternal part of the prepectal ridge (fig. 31, /) which 

 cuts ofif a marginal presternal strip continuous laterally with the pre- 

 episternal areas of the prepectus (figs 26, 30, Ppct). 



The metasternum of Dissosteira (fig. 30 A, Bs^) is wider than the 

 mesosternum and is separated laterally by distinct sutures (r) from 

 the precoxal parts of the metapleura. Its anterior margin, as just ob- 

 served, is extended in a large, median, quadrate lobe which is dove- 

 tailed between the scutellar lobes of the mesosternum. Its posterior 

 edge is broadly emarginate to receive a corresponding extension of 

 the first abdominal sternum (Pc). The median scutellar region of the 

 metasternum, shown diagrammatically at C of figure 30, is suppressed 

 in the same manner as is that of the mesosternum, and the suture of the 

 transverse sternal ridge (^) is here also coincident with the transverse 

 margin of the sternal notch ; but the suture does not extend laterad of 

 the apophyses (sa, sa), and the lateral sternellar lobes are, therefore, 

 4 



