Classification of the Diplopoda. 289 



genera composing the second division, which may be called 

 the Helminthomorpha, and to discuss the relationship that 

 thej bear one with another. As typical genera may be 

 selected Polydesmus, Lysio]_Detalum, Gkordewna^ lulus, and 

 Polyzonium, and the distinguishing characters of each of these 

 are as follows : — 



In Polydesmus the body is composed of not more than 

 twenty segments ; the mandibles have no basilar piece (cardo) 

 and the gnathochilarium has no intergalea (pronientum). The 

 copulatory feet are formed from the anterior pair of the 

 seventh segment, and they are external ; the pedal laminae 

 (tracheal plates. Bourne) are mostly fixed. 



In Lysiopetalum the number of segments is great and 

 variable ; the mandibles have the cardo and the gnathochi- 

 larium the promentum ; the copulatory feet are formed from 

 the anterior pair of the seventh segment, and they are more 

 or less internal j the pedal laminee are all free. 



In Iidus the number of segments is great and variable, the 

 mandibles have the cardo and the gnathochilarium the pro- 

 mentum ; the copulatory feet are formed from both pairs of 

 the seventh segment and are more or less internal ; the pedal 

 laminge are mostly fixed (in a closely-allied genus, IsobateSj 

 they are free). 



In Chordeuma the number of segments is thirty ; the man- 

 dibleff have the cardo and the gnathochilarium the promen- 

 tum ; the copulatory feet are formed from both pairs of the 

 seventh segment and are more or less internal j the pedal 

 laminse are free ; foramina repugnatoria absent. 



In Polyzonium the number of segments is great and variable; 

 the mouth-parts have undergone degeneration ; the copulatory 

 feet are formed from both pairs of the seventh segment and 

 are more or less external ; the pedal laminse are free. 



Setting aside Polyzonium, which in this respect it is not 

 possible to compare, it will be seen from these short descrip- 

 tions that Polydesmus differs from lulus, Lysiopetalum, and 

 Chordeuma in that the mandible is without the cardo and the 

 gnathochilarium without the promentum, and further that 

 in the possession of but one pair of external copulatory feet 

 this same genus presents greater simplicity of organization. 

 Greater simplicity of organization, except where degeneration 

 has occurred, is usually an indication of greater affinity with 

 the ancestral form, and therefore, assuming that the Helmin- 

 thomorpha and the Oniscomorpha have sprung from a com- 

 mon ancestor, we should expect to find the resemblance 

 between Polydesmus and Glomeris greater than the resem- 

 blance between, e. g., lulus and Glomeris ; and this seems to 



