Mr. A. Tulk on the Anatomy 0/ Plialangium Opilio. 157 



are formed, and by which they are bounded from above, laterally, 

 by two pieces which support the maxillary palpi, and below, by 

 the appendages to their internal septum. There are a few scattered 

 spines and bristles upon the upper surface of this joint. Its move- 

 ments are limited to simple ginglymus in a vertical direction, or 

 upwards and downwards, and when at rest, the basal joints lie 

 perfectly parallel with each other, and upon a plane nearly level 

 with the dorsal surface of the cephalo-thoracic shield. The se- 

 cond or terminal joint of the chelicera is articulated to the above 

 at an acute angle. It is broadest behind, where it presents a 

 slight conical projection, and gradually tapers toward its termi- 

 nation, becoming flattened from before backwards, and is there 

 prolonged internally to form a curved horny pincer, dentated and 

 immoveable, and which is opposed to another {d) similarly con- 

 structed but longer, having a single triangular tooth larger than 

 the rest on the middle of its inner edge, and which moves freely 

 in a horizontal direction against the first. Along the upper sur- 

 face of this joint and upon the sides, especially the inner, are rows 

 of short black bristles. When at rest, these joints are bent at a 

 right angle with the basal, and parallel with each other, are sup- 

 ported by the first pair of maxillae. These organs, which, unlike 

 their analogues in the true spiders, are unprovided with any 

 poison-sac, serve merely for grasping the food, and retaining it in 

 contact with the organs of manducation placed upon the under 

 surface of the thorax. Before proceeding, however, to the exami- 

 nation of these latter, it will be best to explain the construction 

 of certain parts, to which allusion has been already made. 



The anterior margin of the cephalo-thorax (PI. III. fig. 3. m) 

 is deflected obliquely downwards for its entire extent, to form a 

 thinner piece, paler in colour than the rest, and grooved along 

 the median line, its free border being crescentic, but prolonged 

 in the middle to form the triangular origin of the vertical sep- 

 tum (5). This septum is laterally compressed, cartilaginous, and 

 membranous to a slight extent upon the margins (PL III. fig. 4. s), 

 but it is not prolonged backwards into the thoracic cavity, so that 

 the muscles, tracheal trunks, &c. passing from the latter into the 

 chelicerse, are in contact with each other internally. The cavities 

 for the reception of the latter organs are incomplete also exter- 

 nally, their outer wall being constituted simply by the basal 

 portion of the maxillary palpus [bp). The horizontal piece, with 

 the posterior margin of which the septum is confluent in a bifur- 

 cated manner below, is of a somewhat elongate and quadrate 

 form. Its upper surface is elevated obliquely forwards from the 

 posterior margin, to form a convex, smooth, transversely oval and 

 regular projection (PI. III. fig. 4. a), slightly overlapping the base 

 of the labrum (/). 



