36 SMITHS()NIAi\ MJSCELLANEOUS COI.I.ECTIONS VOl,. 81 



postoccipital rim, is the iiKjst constant suture of the cranium. The 

 dorsal part of the occipital area before it is termed the occiput (Oc), 

 and the lateral ventral parts the postgenae {Pge). Rarely the occiput 

 and the postgenae are separated, as in Mclanoplus, by a short suture 

 on each side. 



The lateral areas of the cranium, between the occipital suture and 

 the frontal sutures, and separated dorsally by the coronal suture, have 

 been appropriately termed by Crampton (1921) the parietals. The 

 parietal area behind and below the compound eye is the gena (fig. 

 18 B, Ge), that between the eyes is the vertex. The lower marginal 

 area of each lateral wall of the head is commonly marked by a sub- 

 marginal suture (fig. 18 A, B, j-f/.?), which forms an internal ridge 

 strengthening the ventral lateral edge of the cranium (fig. 39 A, 

 SgR). The suture has been termed the '' mando-genal '* suture 

 (Yuasa, 1920, MacGillivray, 1923), but, for grammatical reasons, 

 the writer would substitute the term subgcnal suture, and call the 

 corresponding ridge the siibgenal ridge. The ridge is sometimes known 

 as the " pleurostoma." When an epistomal ridge separates the clypeus 

 from the frons, it unites the anterior ends of the subgenal ridges. 



The true ventral wall of the head is the region between the bases of 

 the mouth parts (fig. 18 D), the median area of which is produced 

 into the variously modified lobe known as the liypo pharynx (Hphy). 

 Anterior to the base of the hypopharynx, and immediately behind the 

 posterior, or epipharyngeal, surface of the lal:)rum and clypeus is the 

 moutli {Mth). The space inclosed by the labrum and the mouth 

 parts is often called the " mouth cavity," but, since it lies entirely 

 outside the body, it is more properly a preoral cavitv. 



The frons, clypeus, and labrum belong to the prostomial region of 

 the head. The frons and clypeus are not always distinct, but when they 

 are separated, the dividing fronto-clypeal groove, or epistomal suture 

 (fig. 18 A, B, c.?), extends typically between the bases of the man- 

 dibles. That the more primitive division of the prostomium, however, 

 is that between the labrum and the clypeal area is evidenced by the 

 fact that the labral retractor muscles always extend from the base of 

 the laljrum to the frontal area (fig. 19, 2, 5). The clypeus, on the 

 other hand, can not be regarded as a mere articular region between 

 the labrum and the frons, secondarily developed into a chitinous plate, 

 as some writers have suggested, because the most anterior of the 

 dilator muscles of the stomodeum have their origins upon its inner 

 surface (fig. 41, jj, 34). The external suture separating the clypeus 

 from the frons appears to be incidental to the development of an 

 internal epistomal ridge (fig. 39 A, B, C, iii?) forming a brace be- 



