TRANSITIONAL FORMS 103 



narrowed at its summit, where it is articulated with the 

 next joint, and from the latter by the well-formed orbits 

 and the entire absence of rudimentary fifth legs.' Those 

 who are always sceptically inquiring for links in the chain 

 of evolution and for the fine gradations which the trans- 

 mutation of species postulates, may be invited to observe 

 in this family the genus Pinnixa, White, in which the 

 fifth legs are often short, the genus Amorpliopus, in which 

 they are rudimentary, and lastly Thaumastoplax and Hexa^ 

 pus, from which they have vanished altogether. 



Some curious facts relating^to the organs of vision 

 in certain members of this tribe are worthy of mention. 

 In the family Ocypodidas the genus Batliyplax, A. Milne- 

 Edwards, 1880, contains but a single species. Specimens 

 taken by the U.S.S. Blalie from depths between four and 

 five hundred fathoms were found to have the eye-stalks 

 very short, almost immovable, and with the corneae not 

 developed. Accordingly the species was named typlilus, 

 ' the blind.' But specimens taken from smaller depths 

 by the Challenger agreed with the others in all respects 

 except just this one, that they possessed small, distinct, 

 terminal corneae. As these specimens were obviously not 

 blind. Mr. Miers named them ' var. oculiferus.' In regard 

 to another genus, also but more doubtfully included in 

 the Ocypodidse, Professor Perrier cites the observation of 

 A. Milne-Edwards, that in Geryon tridens, Kroyer, a 

 species which descends to great depths, the eyes are 

 brilliantly luminous. 



