no. 1831. NEW TROPICAL MILLIPEDS—COOK. 459 



ened surface of segment slightly produced into a broad median lobe 

 and two smaller lateral lobes; apex proper narrow, smooth and white, 

 slightly decurved, scarcely exceeded by the broad median lobe. 



Anal valves horizontal, smooth, slightly convex, with distinctly 

 raised margins, though not very prominent. Preanal scale very 

 broadly triangular rounded, not produced in the middle, with a small 

 setiferous tubercle on each side. 



Seven adult animals were obtained, three males and four females, 

 all in one locality a few miles to the northeast of Mayaguez, along the 

 road toward Las Marias. 



The type of Tridesmus sectilis (Berlin Museum, No. 900) was 

 probably collected near Mayaguez where Krug resided. Iomus is 

 apparently more common than Tridesmus, for no additional speci- 

 mens of the latter genus have been obtained during two visits to 

 Porto Rico. A second specimen has been reported, however, from 

 Utuado, by Professor Silvestri, who describes it as a second species 

 of Tridesmus, T. portoricensis. 1 



Other specimens of Iomus from Barrio Plata and from Bayamon 

 were supposed at first to represent the same species, but more detailed 

 examination shows definite differences that appear to require taxo- 

 nomic recognition. As the specimens from the other localities are 

 all females, only preliminary diagnoses can be offered. 



IOMUS PLATANUS, new species. 



Type-specimen. — Cat. No. 807, U.S.N.M., collected in Barrio 

 Plata, Porto Rico, November, 1899, by O. F. Cook. 



Closely similar to I. incisus, but somewhat more robust, the females 

 attaining 13.5 mm. by 3.7 mm. 



First segment with anterior margin more convex than in I. incisus, 

 and slightly but distinctly scalloped. 



Middle rows of dorsal tubercles distinctly closer to each other than 

 to the outer rows, instead of at equal distances, as in I. incisus. 



These differences may appear rather small, but they are quite 

 definitely shown in all the members of the two series of specimens. 



Eleven adult female specimens and one immature male were col- 

 lected under a rotting log. The male specimen has 18 segments and 

 26 pairs of legs, and measures 5.5 mm. by 1.8 mm. Sternum of seg- 

 ment 7 unmodified; carina? notched as in adult, but the dorsal tuber- 

 cles somewhat less pronounced; repugnatorial pores more distinct. 

 This specimen is of interest as showing that Tridesmus does not repre- 

 sent a younger stage of Iomus, a question naturally suggested by the 

 close external resemblance of the two genera. 



i F. Silvestri, Myriapoda from Porto Rico and Culebra, Bull. Amer. Mus. Nat. Hist., vol. 24, p. 677 

 August, 1908. 



