32 THE SCOLYTID BEETLES. 



although in some cases they may be but faintly or obscurely indicated. 

 The lateral and median sections of the anterior suture separating the 

 prescutum from the scutum have a tendency to curve posteriorly, 

 and especially the median section, which has a very strong tendency 

 in this direction, and is often manifested to such an extent as to t 

 separate the scutum into two lateral sections, as shown in figure 20. 

 The median suture has a reverse tendency, the lateral sections 

 extending anteriorly, the submedian section posteriorly, and the 

 median strongly anteriorly. Thus we often find, as shown in figures 

 20 and 21, that the two sutures overlap and form external longitu- 

 dinal ridges and internal oblique apodemes, with an external median 

 longitudinal impressed area. It appears that the dorsal groove may 

 belong to either the scutellum or prescutum or represent a combina- 

 tion of the two, but for the sake of uniformity in its definition the 

 name scutellar groove is here adopted. The posterior suture is 

 usually distinct in the metatergum, especially in that of Coleoptera, 

 and is much less subject to striking modifications in contour, etc., 

 than the two preceding. Thus, it clearly defines the postscutellum, 

 as in figure 20. 



Transverse divisions. The writer's interpretations of the modifi- 

 cations and position of the four transverse divisions as represented 

 in Dendroctonus are demonstrated in figure 20. 



Prescutum (fig. 20). The area designated as the prescutum is that 

 involving the attachments of the principal sternotergal muscles and 

 the anterior lateral process for the attachment and articulation of 

 the scapular plate. The anterior limit is defined by the prephragma, 

 its posterior limits by the anterior suture and apodeme and the 

 posterior limit of the prescutal lobes, and laterally by the anterior 

 angle or limit of the lateral emargination. The most important 

 features are the prominent prescutal lobes and anterior lobes for the 

 attachment of the depressor muscles, the prescutal disk for the small 

 muscle connecting it with the pleural clavicula, and the triangular 

 prescutal process for the attachment and articulation of the scapular 

 plate. (See, also, figure 21 for the ento tergal characters and anterior 

 apodeme.) 



Prephragma, (fig. 20, B). The prephragma is the median section of 

 the anterior vertical area of the prescutum. Its dorsal and lateral 

 limits are defined by the line of attachment of the intersegmental 

 membrane. The lateral arms in conjunction with the anterior process 

 of the anterior lobe of the presternum are greatly extended ventrally. 



Scutum. The scutum is represented by the large scutal lobes situ- 

 ated each side of the scutellar groove. These lobes are for the ante- 

 rior attachment of the large scutal muscles with the posterior attach- 

 ment to the lateral arms of the postphragma. The lateral margin of 

 the scutum is defined by the lateral emargination and elevated 

 scutellar ridge which terminates in the scutellar process, and poste- 



