232 On the "Seraphim" and its Allies. 



called the mrtastoma, or post-oral plate,* covering the buccal 

 cavity jf the three pairs:}; of simple, many -jointed, and spiny 

 palpi (a, b, c) ; the two broad and oar-like appendages, evidently 

 serving as organs of natation (d d), like the last pair of feet in 

 recent swimming crabs (see woodcut, Fig. D, foot of Platy- 

 OiiicJtus ocellatus). Each pair of organs have their terminal 

 joints serrated at their basal edges, thus serving the offices of 

 foot-jaws and mandibles. 



The thorax and abdomen are composed of twelve joints or 

 segments, which are terminated by a finely-pointed, spear- 

 shaped tail-plate. The first six segments have each a pair of 

 sub-central, spine-like ridges near their posterior margin, pro- 

 bably indicating the points of attachment for muscles within, 

 and are ornamented upon their surface with those peculiar 

 scale-like markings (Fig. h a) which led Agassiz at first to 

 place them in the class of fishes, as already stated. No appen- 

 dages cf any kind have been found belonging to the body seg- 

 ments of these animals, except that about to be described. This 

 plate, seen in situ in our engraving (/, and also in woodcut, Figs. 

 G and K, m), covers the ventral surface of the first thoracic^ 

 segment, and is divided into three parts — 'a median lobe, the 

 extremity of which overlaps the succeeding segment, and two 

 lateral lobes united with it, forming the "conjoined epistoma 

 and labrum" of Messrs. Huxley and Salter. This plate is 

 placed in front, on the under side of the head, in the re- 

 stored figure of P. anglicus, given in Murchison's Siluria, 

 already referred to (see foot note, page 231). Mr. David Page 

 was so fortunate as to possess proof of the true position of this 

 curious segment, which he has figured in the second edition of 

 his Text-Boole of Geology (1859), p. 163, Fig. B, and it is there 

 described as "the anal plate;" but I am not acquainted with 

 any of the Articulata in which that plate occurs in the position 

 indicated ; indeed, there does not appear to be any authority for 

 such a determination. 



On recently examining some scorpions from Smyrna, I was 

 much struck by the analogy existing between the general ar- 

 rangement of their organs and those of Pt&rygotus. The loco- 

 motory appendages, for example, arc all cephalic, and their 



* Serving in place of a lip, or lower j:i\v in higher animals; the true man- 

 dibles and accessory organs of manducation working laterally in nearly all the 

 Articulata. 



f The specimen [s lung upon its buck, having, therefore, the ventral surface 

 displayed to view. 



X I'ivc pairs of organs are known to have existed, see restoration. 



§ This appears really to oonsist of two parts, <>r segments united together; 

 thai next the bead being much narrower than the oilier, thus making the number 

 of segments twelve, as stated, without the tail-plate. This number of segments can 

 he distinctly made out in /'. bitobtu, and both Mr. bliniun and 1 consider it also 

 demonstrable iu aouminatui. 



