PERIDONTODESMUS.—PLATYRACHIDA., 137 
three instead of four teeth, the anterior tooth being large and having a decidedly convex anterior border, 
the total number of teeth on the keel being six and a minute posterior internal tooth instead of seven 
and a small rounded tooth ; the antero-lateral tooth is short and truncate at the apex instead of acute, and 
does not project so far laterally as the second and third teeth; as in P. flagellatus, there are three very 
distinct rows of tubercles on the median area of the terga and only a few upon the keels. 
Total length, 2, about 12 millim., width 2. 
Hab. Mexico, Teapa in Tabasco (H. H. Smith). 
3. Peridontodesmus woodianus. 
Polydesmus (Scytonotus) woodianus, Humb. & Sauss. Rev. et Mag. Zool. 1869, p. 152. 
Polydesmus woodianus, Humb. & Sauss. Miss. Sci. Mex., Myr. p. 52, t. 1. tigg. 18, 13 @ (1872). 
Peridontodesmus woodianus, Attems, Denk. Akad. Wien, lxviii. p. 359, t. 15. fig. 364 (copied) 
(1900). 
The description of this species supplies very few particulars enabling me to compare it accurately with the two 
just described. In two characters mentioned by Humbert and Saussure it differs markedly from them, 
namely, in having the upper side of the keels more thickly tubercular than the median area of the 
segments and in having the lateral angles of the first tergal plate rounded, an epithet by no means 
applicable to the pointed and toothed angle of this plate in P. flagellatus and P. hirsutus. The full 
figure of the type of P. woodianus gives only a vague idea of the general appearance of the animal and 
cannot be trusted for details; the dentition, indeed, is not the same on the right and left keels of 
individual segments. More confidence may probably, however, be placed in the much enlarged figure of 
the dorsal side of one of the segments, which shows that the antero-lateral tooth is minute and sharp, 
and quite different from the corresponding tooth in the two new species here described. It is merely a 
minute denticle, and does not project so far laterally as the second tooth, which is itself surpassed by the 
large bifid pore-bearing tooth. 
Length 12 millim., width 2. 
Hab. Mexico, Eastern Cordillera. 
Fam, PLATYRACHIDA. 
Large, very large, or medium-sized Polydesmoidea, with the caudal process not triangular or cylindrical, but 
broad, squared, oblong, or semicircular, and projecting well beyond the keels of the 19th segment, which 
are small. In all the Central-American species the antennee are short and rather close together, and in 
all genera except Aphelidesmus, which is included with some doubt in this family, the phallopods are 
simple, and the accessory branch, when present, rises near the extremity of the organ as a protection or 
guard to the seminal stile. 
Distribution. Oriental Region eastward from Tenasserim ; South America, Central 
America, and with a few outlying forms in the Southern States of North America ; 
absent from the Australian, Ethiopian, and Holarctic Regions. 
The characters upon which this family rest are not very satisfactory. Nevertheless, 
the numerous genera that have been established are united by a complex of 
structural features which enables them to be distinguished at once from all other 
Polydesmoidea. 
BIOL. CENTR.-AMER., Diplop., October 1909. T 
