RHYSODESMUS. 189 
Tr. Amer. Phil. Soc. xii. p. 224, fig. 55, 1865; also Brdlemann, Mém. Soc. Zool. Fr. 
xiii. p. 101, t. 7. figg. 28, 29, 1900), from the Mississippi. To this genus also belongs 
apparently the species described by Attems as Fontaria laticollis, from Ilinois (Denk. 
Akad. Wien, Ixvili. p. 258, t. 13. fig. 312, 1900). 
Another name that has been introduced, viz. Eurymerodesmus, was proposed by 
Brélemann for a subgenus of Fontaria; the type being the species he identified as 
Fontaria hispidipes, Wood (see Brolemann, Ann. Soc. Ent. Fr. lxv. p. 67, and Mém. 
Soc. Zool. Fr. xiii. p. 101, t. 6. fig. 32, 1900). This species is very nearly allied to the 
type of Fontaria in the structure of its phallopods, but these organs have the coxe 
unraised and the distal segments only lightly arcuate. Moreover, the first segment of 
the legs is unspined. The type of Hurymerodesmus is the species represented by the 
specimens regarded by Broélemann as /*. hispidipes, which may or may not be specifically 
identical with Wood's examples. 
In addition to the above-mentioned groups, one more genus, Aystodesmus, has been 
proposed by Cook for the Japanese species described by Peters as Fontaria martensi. 
This genus, however, does not here concern us; nor do other genera which will 
probably be erected when a comprehensive survey is made of all the species referred to 
Fontaria that have been described from the Chinese area and North America. It will 
suffice here to tabulate the characters of the four North and Central American genera 
above enumerated, in the event of one or more of them being discovered hereafter in 
Central America in addition to Rhysodesmus :— 
a. Phallopod without auxiliary branch; hairy to the apex. 
b. Phallopod strongly arcuate, its coxal segment forming a shelf-like 
lamina; legs with first segment spined . . . 2. . ee OL Fonraria. | 
b'. Phallopod lightly arcuate ; first segment of legs unspined . . . . [ EurymeropeEsmvs. | 
a‘. Phallopod with one or two auxiliary branches ; smooth distally. 
c. Phallopod with two auxiliary branches—one short, pointed, and 
curved, the other straight, as long as the main (seminal) branch, 
and bifid apically . . 2. 2. ee ee ee ee ee ee) [PAcHydeEsus.] 
c'. Phallopod with a single slender auxiliary branch rising on its upper 
(anterior) surface. 2 2 1 ee ee ee ee ee ee.) 6Retysopesuus. 
The genus Rhysodesmus is the dominant Polydesmoid of Central America. Some 
species closely resemble superficially representatives of the genera akin to Leptodesmus, 
but there is very considerable variation in size and shape, owing to specific differences 
in the width and shape of the keels and the degree to which they overlap each other 
in front and behind. As a very general rule in the genus, the keels approximately 
follow the slope of the back, and when the segments are normally contracted overlap 
each other so that they form a continuous shelf-like projection, with uninterrupted 
edge, all along the body. These forms present a stout, compact, and slug-like 
