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38 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 81 



sutures, and by an approximation of their bases, they may constrict 

 the f rons between them. The reversed relative position of the antennae 

 and the compound eyes, as between embryonic and adult stages, comes 

 about through the posterior revolution of the ocular region and the 

 forward migration of the antennae. The antennal socket is generally 

 strengthened by an internal circular ridge on the cranial wall sur- 

 rounding it (fig. 39 A, AR), and the compound eye is likewise en- 

 circled by an inflection of the cuticula close to its base (OR). These 

 ridges and their external sutures set off the so-called ocular and 

 antennal sclerites (fig. iSA, B). 



The posterior, or occipital, surface of the epicranium (fig. i8 C) 

 is usually but a narrow area surrounding the foramen magnum (For) 

 dorsally and laterally, the foramen Iieing normally completed ven- 

 trally by the base of the labium (Lb), or by the neck membrane in 

 which the labium is suspended. When the foramen is small, however, 

 the occipital area often becomes a wide transverse surface on the back 

 of the head, and its ventral, or postgenal, parts may form median 

 processes that sometimes unite into a bridge beneath the foramen, in 

 which case the latter becomes entirely surrounded by chitinous walls 

 (fig. 48 B, C). The occipital suture (fig. i8 B, C, ocs), when present, 

 is generally located a])out where the dorsal and lateral areas of the 

 head wall are reflected upon the posterior surface. It does not seem 

 probable that the occipital suture is a primitive intersegmental line of 

 the head, for, though it lies approximately between the mandibular 

 and maxillary regions, it does not consistently separate the bases of 

 the mandibular and maxillary muscles, and the posterior articulation 

 of the mandible is with the postgena posterior to the lower end of the 

 suture (fig. i8 B, a). As is the case with most of the skeletal grooves, 

 it is pro])able that the occipital suture has no significance in itself, and 

 that it is merely incidental to its corresponding internal ridge, which 

 strengthens the posterior part of the cranium along the line where 

 the dorsal and lateral areas are reflected into the posterior surface. 



In the Machilidae the posterior part of the epicranium is crossed by 

 a prominent suture lying close behind the eyes dorsally (fig. 17 I, 3') 

 and extending" downward on each side of the head to a point on 

 the lateral margin of the cranium between the base of the mandible 

 (Md) and the base of the maxilla (Mx). This suture, therefore, ap- 

 pears to separate the region of the mandibular segment from that of 

 the maxillary segment in the cranial wall, and if it does so, it may be 

 the homologue of the mandibulo-maxillary suture in the phyllopod 

 crustaceans (fig. 17 A, D, y), and of the corresppnding suture in the 

 more generalized malacostracan forms, such as Anaspides. Crampton 



