AND ENDOSKELETAL SYSTEMS OF LIMULTJS. 317 



In front of this, in the median line, is another sclerite, the subfrontal sclerite (Sf). 



Behind the mouth is a large somewhat pear-shaped sclerite, with its broad end 

 directed backwards : this is the promeso-sternite (marked p.m.st in PL LXXVL). 



Behind this come the two apertures leading into the chilaria {mist), identified by 

 Prof. Lankester as the metasternite. Mr. Packard has shown by their development 

 that they do not belong to the series of appendages, and it is obvious enough that they 

 represent the pentagonal or triangular sternal sclerite of the Scorpions. 



Slightly behind these, and high up the sides of the membrane, behind the last ento- 

 coxite, where the membrane rises upwards to join the outward-sloping sternite of chitin, 

 is a sclerite on each side, the lateral sclerite (PI. LXXVI. lat.scl). 



The sides of the prosomatic region are formed simply by the basal joints of the 

 appendages. These basal joints are elongated dorso-ventrally, forming an entocoxite ; 

 and, while the top of this portion, in each case, is attached to the " lateral convexity " 

 (convex chitinous sternal portion of the prosoma), the lower part and the sides are 

 simply held in place by chitinous membrane, which extends all along the ventral 

 median region and up between the basal portion of the limbs to reach the lateral con- 

 vexity (chitinous portion of the prosomatic floor). This holds for the hinder five pairs 

 of prosomatic appendages ; but the first pair is not articulated to any hard part, but 

 simply lies in the membrane. Instead of having an anterior and posterior border to 

 the entocoxite (vide below) there is only one bar to each : and anterior to this, in a 

 line with it, are two sclerites on each side (near the word Cam in PI. LXXIL). 



Each of the last five pairs of thoracic limbs is attached to the lateral convexity 

 (sternal chitinous portion) of the thorax by means of a knob, at the top of the entocoxite ; 

 this articulates with a little hollow in a thickened portion of chitin, whence diverge 

 two rods of harder chitin. This structure, the knob and two rods of hard chitin, 

 continuous with and part of the lateral convexity (chitinous sternite), may be called the 

 coxal pivot or hyper-coxite (see fig. 7, PI. LXXVL). 



The Floor of the Abdomen (meso- and metasoma). — This is continuous with the 

 median floor of the prosoma, and, like it, is membranous. It narrows posteriorly, and 

 is interrupted by six transverse hollows (vn to xn), leading into the six mesosomatic 

 appendages, viz. the genital operculum and five gill-plates. From the hinder edge of 

 each of these hollows there rises on each side, near the middle line, a hollow tendon 

 (ts 1 to ts e ), continuous posteriorly with the stigmata on the base of the abdominal 

 appendages, and at their anterior ends having each a muscle inserted. These tendons 

 and their stigmata will be found described and figured in Prof. Lankester 's Memoir 

 " Limulus an Arachnid." 



Between each pair of these " tendinous stigmata " is situated in the middle line, on 

 the posterior border of the transverse hollows in the floor, a small rectangular carti- 

 laginous " entochondrite," to which muscles are attached (s 1 to s 6 ). 



