460 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.40. 



IOMUS OBLIQUUS, new species. 



Type-specimen. — Oat. No. 808, U.S.N.M., collected near Bayamon, 

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



Closely similar to /. incisus and /. platanus, but slightly more 

 slender, the female measuring about 13.5 mm. by 3.4 mm. 



First segment with anterior margin more transverse in the middle 

 than in I. incisus, but also more distinctly scalloped than in /. 

 platanus. 



Dorsal tubercles of middle rows distinctly closer to each other than 

 to the outer rows, but the posterior tubercles of the middle rows much 

 farther apart than the anterior, so that the rows appear distinctly 

 oblique, or converging forward, especially on segments near the 

 middle of the body. On the anterior segments the anterior tubercles 

 of the middle rows are distinctly enlarged. The same tendency is 

 shown to a slight extent in /. incisus, while in /. platanus the middle 

 tubercle of the inner rows are often somewhat larger than the others 

 as in the outer rows. In the present species the enlarged anterior 

 tubercles are united with two broad median lobes of the margins of 

 the subsegments. There are two somewhat smaller lobes on each 

 side separated by notches like those of the margins of the carina?. 

 The raised margins project somewhat forward and are more prominent 

 in this species. The surface of the segments shows a suggestion of 

 division into areas by very slight impressed lines. The carinas appear 

 shorter and somewhat more widely separated than in /. platanus, 

 and the marginal incisions are somewhat deeper and more open. 



The wider separation of the posterior tubercles of the inner rows 

 in this species may be considered as an approximation to the arrange- 

 ment of the outer rows, in the same way that the tendency to the 

 enlargement of the middle tubercles of the inner rows in /. platanus 

 approximates the specialization of the middle tubercle of the outer 

 rows. 



A single female specimen was found on limestone rocks near Baya- 

 mon. The color is a deep brown, not completely black, as in the other 

 species, and especially in I. platanus. 



SYNOPSIS OF WEST INDIAN GENERA OF CHYTODESMID^E. 



The Chytodesmidse are more nearly related to the African families 

 Stylodesmidre and Hercodesmidoe than to the true Cryptodesmidse 

 of South America. The gonopods agree with those of the African 

 families in having the basal joint large and clypeate to contain the 

 small folded terminal joint, but the segments lack the huge dorsal 

 processes that characterize the African families. The slight develop- 

 ment of the dorsal tubercles gives these West Indian genera a super- 

 ficial resemblance to a third African family, the Pterodesmidse, but 

 the gonopods are of entirely different patterns in the two groups. 



