1887.] 



ON CYCLESTHERIA HISLOPI. 



ID 



excavated, to receive the powerful adductor muscles. Above it 

 gradually tapers to a sharp point, which is movably attached 

 to a small chitinous strip of the outer skin, at a short distance 

 below the occipital notch (see Pl. III, fig. 5, M). The mastica- 

 tory part forms almost a right angle with the body (see Pl. IV, 

 fig. 3), being incurved so as to meet the corresponding part on 

 the other mandible; it is somewhat appressed toward the tip and 

 terminates with an elliptic molar surface (see fig. 2), which exhib- 

 its a finely fluted sculpture, produced by niimerous transverse 

 rows of minutely denticulate ridges densely crowded together. 

 As to the mnscnlar apparatus of the mandibles, besides the strong 

 adductor muscles, filling up the concavities of their bodies and 

 connected together by a chitinous lamellar tendon traversing the 

 segment, two other muscles are seen extending from the dorsal 

 side to the body of each mandible (see Pl. III, fig. 5). Of the 

 latter, representing the rotatory muscles, the anterior originates 

 from the preoral part of the head, at a short distance in front 

 of the occipital notch, and joins the anterior edge of the man- 

 dible above the middle; the posterior, by far the stronger of 

 the two, originates with numerous converging bundles along the 

 whole convex dorsal surface of the postoral part of the head, joining 

 the posterior edge of the mandible throughout its upper half. 



b. The Cervical Division, or Neck. 



This division. which in other Crustacea is generally adduced 

 to the head, cannot properly in the present form be referred 

 either to that or the succeeding part, the trunk, forming, as it does, 

 a distinctly defined subdivision of the body, chiefly characterised 

 by its performing the connection between the shell and the body. 

 This connection is effected partly by the strong adductor muscle 

 of the shell. which traverses the low^er part of this division, 

 and partly by the above-mentioned dorsal prolongation. The 

 latter part contains within it the anterior extremity of the heart, 

 and exhibits on either side two slender di ver gin g muscles running 

 from the dorsal part of the shell obliquely downward, the 



2* 



