428 K. I. POCOUK. 



wellkuown resemblances with regard to the clistributiou 

 of the stigmata, and the alternate development of larger 

 and smaller tergal plates. 



In both forms the first, third, fifth, seventh, eighth, tenth, 

 twelfth, and fourteenth terga are large, the second, fourth, 

 sixth, ninth, eleventh, thirteenth, and fifteeuth small, and 

 stigmata are situated beneath the lateral margins of the 

 third, fifth, eighth, tenth, twelfth, and fourteenth. On the 

 other hand, the difference consists in the circumstance that 

 whereas in Lithobius the genital and anal somites succeed 

 the fifteenth, in Scolopendra no fewer than six somites 

 — repeating the characters of those that precede them, 

 the sixteenth, eighteentli, and twentieth being large and 

 stigmatiferous, the seventeenth, nineteenth, and twenty- 

 first small and astigmous — are intercalated between the 

 fifteenth and the genital. 



These facts forcibly suggest the conclusion that the 

 fifteen somites of Lithobius are homologous to tlie 

 anterior fifteen somites of Scolopendra, and that the 

 reduction in the number of somites in the former is due 

 to the excalation of six somites betAveen the fifteeuth and 

 the genital, or to the failure to develop them from the 

 embryonic "caudal lobe," the genital somite being pre- 

 sumably homologous in the two forms, and, in conjunction 

 with the anal somite, representing in the adult the posterior 

 portion of the " caudal lobe " in the embryo. 



There is an attractive simplicity about this view of the 

 case which formerly induced me to hold it as a working 

 hypothesis. But the discovery of Craterostigmus puts 

 a quite different complexion on the whole question. 



In the first place, the coi'respondence with respect to 

 numbers of stigmata and legs between Craterostigmus 

 and Lithobius, and the position of the stigmata above the 

 bases of the third, fifth, eighth, tenth, twelfth, and four- 

 teenth legs on each side, fully justifies the supposition that 

 the stigmata and legs are strictly homologous in the two 

 forms. Indeed, it is not easy to see what other opinion 



