10 Mr. R. I. Pocock on the 



between the fourth segments of tlie limbs in TJielyphonus and 

 Scorpio — segments Avhich, according to Mons. Gaubert, are 

 not homologous, that of Thelyphonus being a patella while 

 that of Scorpio is a tibia — seem to me to be serious obstacles 

 to an otherwise plausible theory. 



So far as the mandible is concerned, there is not much that 

 need be repeated here. With the possible exception of some 

 Acari, the greatest number of segments for these appendages 

 is three. Three are found in the Scorpiones, most Opiliones, 

 and tlie Palpigradi ; the basal one, however, has disappeared 

 in the Pedipalpi, Araneaj, Pseudoscorpiones, and some Opi- 

 liones. The Solifugse, too, are usually said to resemble the 

 Araneffi in having two-segmented jaws ; but in reality they 

 appear to have the primitive number, namely three ; the 

 basal one, however, has hitherto never been recognized, on 

 account of its fusion with the cephalic shield. It is, never- 

 theless, a more or less distinctly defined sclerite to which the 

 rest of the jaw is articulated. 



A further peculiarity in the mandibles of the Solifugai is 

 the inferior position of the terminal segment or fang with 

 respect to the apophysis of the penultimate segment, to which 

 it is opposable. Also in the male there is a peculiar process 

 on the penultimate segment. Peculiar processes are also 

 found on the same segment of the mandible in both sexes of 

 the Pseudoscorpiones ; the movable dactylus, too, is almost 

 inferior. Mainly on these two points of similarity, Mons. 

 Simon has expressed the belief that the affinities of the 

 Solifug^ are with the Pseudoscorpiones — an hypothesis which 

 is further supported by the absence of a cephalothoracic 

 sternum and a close similarity in the number of abdominal 

 segments. 



Another system of organs, which no doubt, if more were 

 known of their structure, would throw light upon the affinities 

 of some of the orders of Arachnida, is the eyes. 



The ancestral form we believe to have been furnished with 

 two sets of visual organs, which differed in structure and 

 mode of development. These median and lateral eyes are 

 well shown in the Scorpiones, most of the Pedipalpi, and 

 most Araneffi. But there is a marked tendency in many of 

 the ' higher ' forms to the disappearance of some or all of the 

 eyes. In the higher Opiliones the median only are retained. 

 This also appears to be the case in the Solit'uga3. In the 

 Pseudoscorpiones, on the contrary, the median seem to have 

 disappeared and one or more of the lateral often retained. But 

 until the histology and embryology of these organs has been 

 worked out in this group, in the Acari and in StylocelluSj 



