PLATYDESMUS. 43 
The two species of this genus described by Lucas, namely, Platydesmus polydesmoides 
and Piestodesmus moreleti, were sufficiently distinct to justify fully at the time this 
author’s view as to the generic value of their structural characters, de Saussure’s 
statement to the contrary notwithstanding. The principal difference between the 
types of Platydesmus and Piestodesmus, apart from the much greater width of the 
body in the latter, lies in the form of the first tergal plate, which in Piestodesmus is 
greatly expanded in front so as to cover the head completely, whereas in Platydesmus 
it is widely excavated, leaving the summit of the head wholly exposed. The species 
described and seen by de Saussure seem in no respect to lessen the value of this 
structural character by supplying an intermediate stage between the two forms of 
the tergal plate in question. So far as this structure is concerned, Platydesmus 
mexicanus is a genuine Platydesmus, agreeing with polydesmoides and differing 
essentially from Piestodesmus moreleti. Nevertheless de Saussure’s union of the two 
is justified, and shown to be correct by the evidence furnished by the species in the 
Godman and Salvin Collection. Amongst the Diplopods collected at Omilteme by 
Mr. H. H. Smith, I find the two species of Platydesmus described below as P. hirudo 
and P. mesomelas, which, with P. marmoreus, serve to bridge over completely the 
interval between the extreme forms first named by Lucas. 
I am unable to find any justification for Bollman’s statement (Bull. U.S. Nat. Mus. 
no. 46, pp. 137 & 188, 1893) that the type of Piestodesmus differs from that of Platy- 
desmus in having two ocelli instead of one on each side of the head. Lucas describes 
the species as having two—that is to say, one pair of ocelli. 
Bollman also states that the genus Brachycybe, H. C. Wood (Proc. Acad. Phil. 1864, 
p. 187; Trans. Amer. Phil. Soc. 1865, p. 230), agrees in all points with Platydesmus, 
except in being eyeless; and this feature he did not consider to be of generic value. 
Silvestri, however, kept the two distinct on account of it; but that a specific rather 
than a generic importance should be attached to the presence or absence of these 
organs is, in my opinion, established by the discovery of Platydesmus guatemalensis, 
which seems to differ from the typical and other species of Platydesmus only in 
this particular, apart, that is to say, from other characters of admittedly specific 
significance. Still I hesitate to follow Bollman and Brélemann in adding Brachycybe 
to the synonymy of Platydesmus. Bollman’s description of P. lecontei, the type of 
the genus, supplies no data justifying the separation of the two; but Brolemann’s 
diagnosis (Mém. Soc. Zool. Fr. 1900, p. 110, t. 57. figg. 69-77) assigns to this species 
two characters which are not’ found in any of the Central-American species of Platy- 
desmus available to me for examination. These are: (1) the reduction in width of the 
sternal areas so that even in the mid-region of the body the coxal segments of the 
appendages of the right and left side are mesially in contact; and (2) the presence 
upon the sterna of a cariniform tubercle, which projects forwards in the middle line. 
G 2 
