AND ENDOSKELETAL SYSTEMS OF LIMULUS. 315 



carapace. Behind the last of these the edge is continued into a point similar to 

 that of the recurved hinder portion of the prosomatic carapace. Behind the arch in 

 the anterior border is a median arched portion of the carapace, transversely marked 

 by six very slight depressions ; between each of these, at the side of the arched part, is a 

 pair of pits, the point of invagination of six pairs of " entapophyses " (Owen). Outside 

 this line the carapace slopes downwards to the edge. Behind the last pit of invagination 

 is a smoother part, which extends a short way backwards, and is continued outwards to 

 form the posterior portion of the edge, which ends in a point. The hinder edge of the 

 abdominal carapace is scooped out ; in the bay thus formed a postanal spine is articu 

 lated by means of a strong membrane. 



3. The Postanal Spine itself consists of a long tapering piece, triangular in section, 

 with the apex of the triangle upwards. It is the hinder portion of the typical 

 "telsonic" segment, and is the exact equivalent of the Scorpion's "sting." At its 

 articulation with the body it has a dorsal process, which curves slightly forwards, 

 and has the strong articulating membrane attached to it. The basal piece spreads 

 out, and is likewise continued slightly forwards, and has also the strong membrane 

 attached (PL LXXIII. sp). 



b. Internal Aspect. (Plate LXXVI. fig. 1.) 



1. The Prosomatic Carapace is thus concave when seen from below and within, 

 running downwards in front and at the sides to join the sternite. Behind, from the 

 vertical border, rise the entapophyses; these are strong processes, triangular in trans- 

 verse section at their base, but flattened and broadened at their free ends ; they are 

 directed forwards, downwards, and slightly inwards (enf). To these structures various 

 muscles are attached. 



From each of these entapophyses there runs forward a ridge (seen as a depression 

 from without) with slight minor ridges branching at the sides; outside this are attached 

 the main coxotergal muscles, each attachment being roughly separated by a slight ridge 

 from its neighbours (25, 26, 28, &c); within the ridge are attached other muscles 

 from the coxae, and from the plastron and from the abdominal appendages (18, 51, 

 52, &c). Lying along the posterior edge of the carapace is a curious network of 

 chitin (PL LXXVI. N) ; this is continued forwards along the line of the lateral eyes. 



2. The inner surface of the Abdominal (Meso-metasomatic) Carapace is far less 

 extensive. It is in front continuous with the hinder portion of the prosomatic 

 carapace, and thence backwards this surface narrows till behind it has only the width 

 of the postanal spine. 



At the sides the floor of the mesosoma rises upwards, meeting it above the meso- 

 somatic appendages just beyond the line of the entapophyses ; thence the two, fused 

 together, continue outwards as a thin plate for a short way. This then rapidly thickens 



